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PREPARATION OF POLYSTYRENE WITH HIGHER Tg BASED ON TRIPLE HYDROGEN BOND INTERACTION

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Abstract............................................................... ......... .........2

Abstract in Chinese ... ................................................................4

1.1 Glass transition temperature.............................................................5
1.2 Styrene and Maleimide copolymer....................................................14

1.3 Melamine and chemistry .............................................................23

1.4 Diaminopyridine molecule..............................................................26

1.5 the thesis work...........................................................................27

Chapter 2 Experimentation Section.............................................31

2.1 Materials.................................................................................31

2.2 Instruments..............................................................................31

2.3 Synthesis of random copolymers of styrene and maleimide......................32

2.4 Synthesis of Blends of Styrene/Maleimide copolymer and melamine...........32

2.5 Synthesis of Blends of the copolymer and 2,6-diaminopyridine.................33

Chapter 3 Results and discussion................................................34

3.1 Characterization of copolymer of styrene and maleimide..........................34

3.2 Copolymerization of styrene and maleimide........................................33

3.3 Blends of the styrene/maleimide copolymers and melamine............................38

3.4 Blends of the copolymer and 2,6-diaminopyridine..................................43

Chapter 4 Conclusion.................................................................46

Reference...................................................................................47

PREPARATION OF POLYSTYRENE WITH HIGHER Tg BASED ON HYDROGEN BOND INTERACTION

ABSTRACT

Polystyrene is one of a common polymer. But its glass transition temperature (Tg) is only 100^oC which leads to limit its applications. In the thesis, improvement of its Tg was done via increase of its chain interactions to restrict the chain flexibility.

Styrene was copolymerized with maleimide, imide of which act as a hydrogen-bond interaction site in the copolymer, by free radical polymerization method in DMSO solution at different temperature, and different ratio of styrene to maleimide, and different reaction time, using different amount of AIBN as an initiator. To make the imide distribute randomly in the chains, maleimide was dropped slowly during the polymerization. Melamine and diaminopyridine were selected to be as interaction molecule because they can form the triple hydrogen bond with imide in the copolymer. Addition of melamine or diaminopyridine into the copolymer results in a dramatic increase of Tg. Polystyrene with imide molar concentration of 5.05% has Tg of 122^oC in the presence of melamine, 22^oC higher than polystyrene, which will extend application of polystyrene. At high ratio of melamine to imide, two Tgs are observed, one is higher than 122^oC but another much lower. The existence of two Tg is due to the fact that free melamine is acting as plasticizer.

Diaminopyridine also increase Tg of polystyrene, but not effectively than melamine owing to lack of crosslinking. In this case, the blend of Diaminopyridine with polystyrene containing imide looks like a copolymer of styrene with a big monomer, complex of maleimide and Diaminopyridine.

The secondary interaction characteristics between melamine or Diaminopyridine with polystyrene containing imide is confirmed by their blends readily soluble in DMSO and in CH₂Cl₂.

Chapter 1 Introduction

1.1 Glass transition temperature

Many plastics lose their strength at relatively low temperature. Continuous-service-temperature comparison of plastics reveals that most common plastics can endure temperature more than 150^oC when under low or no stress. Glass transition temperature or Glass point (T_g) is the point at which polymers act as glass or become viscous liquids. That's a very important factor for polymers to evaluate their processing and application performances.

The Glass transition is a reversible change that occurs when a resin polymer is heated to a certain temperature (T_g), resulting in a sudden change or transition from rigid polymer to a flexible, rubbery material or a viscous liquid. When the polymer is cooled below this temperature, it becomes hard and brittle, like glass. Certainly there are a few polymers used above their Glass transition temperatures, however majority of polymers are used below. Popular hard plastics like polystyrene (T_g=100^oC) is used below their glass transition temperatures; that is its glassy state. Their Glass transition temperatures are well above room temperature, both at around 100^oC.

Rubbers elastomers like polyisoprene and polyisobutylene are used above their Glass transition temperatures; that is in the rubbery sate where they are soft and flexible.

The glass transition differs from the melting transition by the fact that the former is a transition which happens to amorphous polymers and the latter is a transition which occurs in crystalline polymers. But even crystalline polymers will have some amorphous portions; this portion usually makes up 40-70% of the polymer sample. This is why the same sample of a polymer can have both a Glass transition temperature and a melting temperature. But only the amorphous portion undergoes the glass transition and only the crystalline portion undergoes melting. This change in mobility with temperature happens because heat is really a form of kinetic energy.

The exact temperature at which the polymer chain undergoes this big change in mobility depends on the structure of the polymer. A polymer chain that can move around fairly easily will have a very low Tg, while one that doesn't move so well will have a high one. The more easily a polymer can move the less heat it takes for the chains to commence wiggling and break out of the rigid glassy state and into the soft rubbery state. A given polymer sample does not have a unique value of T_g because the glass phase is not at equilibrium.

1.1.1 Factors governing Tg

The height of the glass-rubber transition temperature is, in the first instance, governed by the competition between thermal motion and the attraction forces between the chains.

The thermal motion is depend on the freedom of the chain to undergo changes in conformation .When this freedom is higher, the chain is subjected to a stronger thermal motion than a chain which, e.g. as result of hindrance in rotation, is more rigid, the chain stiffness plays an important role.

1.1.2 Chain flexibility

Higher chain stiffness results from a smaller number of possible chain conformations; this can be caused by:

The increasing size of the side group effects a decrease of chain flexibility and an increase of T_g.

1.1.3 Chain interactions

The strongest of molecular interaction are the dipole forces. Their effect on Tg is illustrated by the series PP, PVC and PAN, in which the chain mobility hardly varies because the side groups are of about equal size, but in which, in the order of sequence mentioned, the dipole interaction increase.

Interaction can be decreased by increasing the distance between the chains, for instance with long side chains, which lower Tg. This effect appears to be greater than the increase of chain stiffness, as shown in the examples below:

Approximate glass transition temperatures and melting point of a few polymers are shown below:

Polymer	LDPE	HDPE	PP	PVC	PS	PAN	PTFE	PMMA	PMMA
T_m(^oC)	110	130	175	180	175	>200	330	180	30
T_g(^oC)	-110	-110	-20	80	100	95	-110	105	-70

1.1.4 Intermolecular interaction

Although all such forces arise from the same fundamental source i.e., interaction of negatively charged electrons and positively charged nucleus yet they differ in magnitude, effective range and mode of operation. Usually they are much smaller than the forces responsible for chemical bonding.

1.1.5 Van Der Waals interactions

These interactions arise due to transfer polarization of neutral molecules and are also known as London forces. Usually neutral molecules have balanced number of negative electrons and positive charge on the nucleus. Yet since electrons are in motion, the centre of density of negative charge may not coincide with the centre of density of positive charge continuously. A molecular thus acquires an electric dipole and can exert an attraction for other similar molecules. Such interaction is known as van Der Waals interaction.

A polarized molecule may induce the electric dipole in a neutral molecule. However such polarized molecule continually reverts back to neutral state and dipole is only transient. The greater the number of electrons in a molecule and farther their distance from nucleus, the greater will be the case of polarization and consently stronger Van Der Waals forces. These forces vary inversely with the seventh power of the distance between molecules.

V.F. á 1/d7

Where V.F. is Van Der Waals forces and d is distance separating the molecules. They are effective only over short intermolecular distances.

1.1.6 Dipole-Dipole Interactions

Unequal sharing of electrons in covalent bonds results in bonds dipoles and their magnitudes are indicated by the bond moments. As may be expected the bond dipoles in different molecules attract each other resulting in dipole-dipole interaction. These forces (D.F,) are governed by the expression: D.F. á 1/d⁴

Where d is the distance between molecules, thus these forces also are effectives only over short distances but have larger range than Van Der Waals forces.

1.1.7 Hydrogen Bonds

It has been observed that when a hydrogen atom in a compound is bonded to a highly electronegative atoms such as N, O, F, then marked differences are observed in its usual properties like boiling point, solubility etc. For example the boiling point of organic compounds usually increases with increase in molecular weight but, though ethyl alcohol C₂H₅OH (b.p.78.2^o) and dimethyl ether CH₃-O-CH₃ (b.p. -24.9^o) have the same molecular weight, yet there is large difference in their boiling points.

The chemical properties of these compounds also differ as compared to similar compounds not having hydrogen attached to N, O, F.

It is argued that when hydrogen is attached to such electronegative atoms the bonding electrons are drawn strongly towards the electronegative atom creating a dipole in the molecule. The hydrogen atom therefore, acquires a small positive charge and becomes extraordinarily capable of attracting a negatively charged atom of a molecule. This attraction results in association of such molecules though the H-atom known as Hydrogen Bond. This is represented by a dotted line. It has much less strength than covalent bond and is essentially the result of electrostatic interactions, delocalization effects and dispersion effects.

Hydrogen bonds are attractive interactions between a positively charged hydrogen atom bonded to an electronegative element (the donor:

), and a negatively charged atom with a lone pair of electrons (the acceptor:

)

Table1-2 Functional group that can form hydrogen bonds, arranged by element

The strongest hydrogen bonding is formed between a strong donor (like F-H and O-H in acid) and a strong acceptor.

The type of H-bonding resulting in association of two or more molecules of the same or different compound is known as Intermolecular hydrogen bonding. Intermolecular association trough hydrogen bond results in unusually high boiling points of the liquids. Thus, the high boiling points of water, alcohols, amines and acids as compared to monomeric molecules of comparable molecular weight may be explained on the basis of H-bonding.

Intermolecular Hydrogen bonding is the formation of H-bonging within the molecule itself. Ethylacetoacetate, salicylaldehyde and o-nitrophenol are example of this type.

1.2 Styrene and Maleimide copolymer
Polystyrene is one of a common polymer. It is very easy to produce and proceed, so very cheap, and has majority of properties for usage in common life. However, its glass transition temperature (Tg) is only 100 degree Celsius, which leads to limit its applications.

Polymers with high glass transition temperature are attractive for industrial polymer science because of their strong economic rewards that may arise from their potential application^[1]. As mentioned above, two factors governs Tg of polymers, chain flexibility and chain interaction. Copolymerization is a best way to change both of them. In the case of copolymers, the final value of a given property; e.g. the melting point or the glass transition temperature does depend on both of monomer structures and the composition of them, also the others^[2]. The existing methods used to improve Tg of Polystyrene are the copolymerization and control of its configuration. Incorporation of a few of another stiff monomer shows less improvement in Tg of polystyrene, because the Tg is a function of content of stiff monomer. More content of stiff monomer makes Tg of styrene copolymer higher, meanwhile many good properties, for instance, stiffness, transparent, and processing property, will be lost. Isotactic polystyrene has very high Tg(above 220^oC). However, it is difficult to process and, furthermore, it is gotten by much more complicated coordination polymerization route, not by the easy free radical polymerization.

Styrene molecule or derivates are chemically modified by free radical polymerization to obtain new products with various potential applications and properties^[3]. Copolymerization of maleimides with styrene provides the possibility of synthesizing higher and thermally stable polymers. In addition to that the processability of maleimide polymer can also be enhanced by the incorporation of more flexible units within the polymer backbone ^[1].

Styrene-Maleimide copolymer (SMA) have been found to have versatile applications in many industries ranging from aerospace to the microelectronics field^[2]. During the past several years many reports and researches in the free radical copolymerization of styrene with maleimide have emerged^[2-5].

1.2.1 General description

When a polymer is made by linking only one type of small molecule, or monomer, together, it is called a homopolymer. When two different types of monomers are joined in the same polymer chain, the polymer is called a copolymer. Two monomers A and B can be made into a copolymer in many different ways.

When the two monomers are arranged in an alternating fashion, the polymer is called an alternating copolymer:

In a block copolymer, all of one type of monomer are grouped together and all of the other are grouped together. A block copolymer can be thought of as two homopolymers joined together at the ends:

When chains of a polymer made of monomer B are grafted on to a polymer chain of monomer A we have a graft copolymer:

Styrene, also known as vinyl benzene is an organic compound with the chemical formula C₆H₅CH=CH₂. Under normal conditions, this aromatic hydrocarbon is an oily liquid. It evaporates easily and has a sweet smell, although high concentrations confer a less pleasant odor. Styrene is the precursor to polystyrene, an important synthetic material. Styrene Monomer is the raw material for polystyrene and EPS, accounting for approximately two thirds of total styrene monomer production. The remaining styrene monomer is used as a feedstock in the production of SAN, ABS and the unsaturated polyester resins, SBR and polymer latex. Major downstream styrene markets (polystyrene and ABS) are under pressure as a result of oversupply and interpolymer competition, although rationalization of older, smaller styrenics units should help balance out the market.

Maleimide and its derivatives are prepared from maleic anhydride by treatment with amines followed by dehydration. A special feature of the reactivity of maleimides is their susceptibility to additions across the double bond either by Michael additions or via Diels-Alder reactions.

A Copolymer is a polymer chain made up of two monomers units, say A and B. When produced by copolymerization, the copolymer chain will comprise a distribution of sequence lengths (S)which will depend upon the monomer feed ratio [A]/[B] and the reactivity ratio (r_A and r_B)which reflect the inherent tendencies of a radical to react with its own monomer relative to the co-monomer.

Therefore, because of the feed ratio, the polymerization of two or more monomers will result either to random copolymers, alternating copolymer and block copolymers ^[6,7].

If a feed ratio of

is taken, then the reactivity ratios will indicate the inherent tendency of a system to produce particular sequence length distributions and hence the characteristic average sequence lengths. As an example, for a perfectly alternating copolymer, r_A=r_B=0 and the monomers alternate along the chain. Alternatively, for a completely random copolymer r_A=r_B=1 and S_A=S_B=2

One of the most common and useful reaction for making polymers is free radical polymerization.

Copolymerization of maleimide with styrene monomers results in a polymerization mechanism occurring via a charge-transfer complex or via the penultimate model, but it is obvious that polymerization of an electron rich monomer (styrene) with an electron poor monomer (maleimide) leads to a, predominantly, alternating copolymer [8,9].

Fig 1-1 charge transfer complex during the polymerization of styrene-maleimide copolymer

1.2.2 The initiator of Styrene-Maleimide copolymer

The whole process starts off with a molecule called an initiator. This is a molecule like benzoyl peroxide or 2,2'-azo-bis-isobutyrylnitrile (AIBN).

What is special about these molecules is that they have an uncanny ability to fall apart, in a rather unusual way. When they split, the pair of electrons in the bond which is broken will separate. This is unusual as electrons like to be in pairs whenever possible. When this split happens, we're left with two fragments, called initiator fragments, of the original molecule, each of which has one unpaired electron. Molecules like this, with unpaired electrons are called free radicals.

The carbon-carbon double bond in a vinyl monomer, like ethylene, has a pair of electrons which is very easily attacked by the free radical. The unpaired electron, when it comes near the pair of electrons, can't help but swipe one of them to pair with itself. This new pair of electrons forms a new chemical bond between the initiator fragment and one of the double bond carbons of the monomer molecule. This electron, having nowhere else to go, associates itself with the carbon atom which is not bonded to the initiator fragment. You can see that this will lead us back where we started, as we now have a new free radical when this unpaired electron comes to roost on that carbon atom. This whole process, the breakdown of the initiator molecule to form radicals, followed by the radical's reaction with a monomer molecule is called the initiation step of the polymerization.

This process, the adding of more and more monomer molecules to the growing chains, is called propagation.

Because we keep remaking the radical, we can keep adding more and more ethylene molecules, and build a long chain of them. Self-perpetuating reactions like this one are called chain reactions.

Coupling is one of two main types of termination reaction. Termination is the third and final step of a chain-growth polymerization.

1.3 Melamine molecule and chemistry

As we known, the physical properties of polymers are significantly altered upon addition of low molecular weight compounds, e.g. the addition of plastizers, which increase chain mobility, and enhance the processability of polymeric materials^[4]. However, a low molecular weight compounds is easy to distribute in polymers and have a strong interaction with polymers in some cases. It's known that the used of secondary interaction(such as hydrogen bond, ion-ion, dipole-dipole or Van Der Waals forces ) is also a well-accepted strategy to enhance miscibility of immiscible polymers, and miscible polymers mixture based on hydrogen bonding^[5]. Most significantly, secondary interaction for the self-assembly of macromolecules has been investigated more recently in detail for liquid crystallinity, rotaxanes, catenanes and amphiphilic structures ^[5]. Recently, Lehn et al. introduced the concept of using well-defined secondary interactions based on hydrogen bonding in the synthesis of liquid crystalline polymer, in which the repeating units are linked by triple-hydrogen bonding instead of by covalent bonding^[6].

Ronald F.M. Lange et al. have studied the interaction between the alternating copolymer of styrene/maleimide with unsubstituted melamine . They observed that addition of melamine results in a dramatic decrease of T_g up to melamine concentration of 20%(w/w) at which the T_g remains constant at around 215^oC^[5,7].

1.3.1 Structure of Melamine

Melamine is an organic base with the chemical formula C₃ H₆ N₆, with the IUPAC name 1,3,5-triazine-2,4,6-triamine. It is only slightly soluble in water. Melamine is a trimer of cyanamide. Like cyanamide, it is 66% nitrogen (by mass) and provides fire retardant properties to resin formulas by releasing nitrogen when burned or charred.

1.3.2 Chemistry of Melamine

It is well-known that melamine forms a 1;1 crystalline complex [5,7,12]with cyanuric acid. The structure of this complex is proposed to be an infinite two dimensional lattice as is showing in the figure 5

Fig 1-4 infinite two dimensional lattices, proposed for the 1:1 complex of melamine and cyanuric acid.

This 1:1 melamine-cyanuric acid lattice has been a source of inspiration in the development of supramolecular chemistry and the theory of secondary interaction^[7,12].

Various model studies (e.g. complexation of unsubstituted melamine with a low molecular weight imide as well as with various imides containing polymers) have been performed to mimic this triple hydrogen bond formation^[12].

However, few attempts have been made to mimic this triple hydrogen bond formation using high molecular polymeric materials. In order to mimic this triple hydrogen bond formation only alternating copolymers of SMA have been used in the synthesis of blend between SMA with either melamine or 2.4-diaminotriazine^[12]

1.4 2,6-diaminopyridine molecule

2,6-diaminopyridine is analogous melamine. It can also form the triple hydrogen bond with SMA.

Fig 1-5 Potential precursors of the triple hydrogen bonded Styrene Maleimide couple

PYRIDINE is a heterocyclic aromatic tertiary amine characterized by a six-membered ring structure composed of five carbon atoms and nitrogen which replace one carbon-hydrogen unit in the benzene ring (C₅H₅N). The simplest member of the pyridine family is pyridine itself. It is colorless, flammable, toxic liquid with a unpleasant odor, miscible with water and with most organic solvents, boils at 115 ^oC. Its aqueous solution is slightly alkaline. Its conjugate acid is called pyridinium cation, C₅H₅NH⁺, used as a oxidation agent for organic synthesis. Pyridine is a base with chemical properties similar to tertiary amines. Nitrogen in the ring system has an equatorial lone pair of electrons that does not participate in the aromatic pi-bond. Its aqueous solution is slightly alkaline. It is incompatible and reactive with strong oxidizers and strong acids, and reacts violently with chlorosulfonic acid, maleic anhydride, oleum, perchromates, b-propiolactone, formamide, chromium trioxide, and sulfuric acid. Liquid pyridine easily evaporates into the air. If it is released to the air, it may take several months to years until it breaks down into other compounds. Usually, pyridine is derived from coal tar or synthesized from other chemicals, mainly acetaldehyde and ammonia.

Pyridine and its derivatives are very important in industrial field as well as in bio chemistry. 2,6-Pyridinediamine is used as an intermediate for the synthesis of analgesic drugs. Phenazopyridine is an example derived from 2,6-Pyridinediamine.

1.5 The thesis work

The use of secondary interactions a well-accepted strategy to enhance the miscibility of immiscible polymers, and miscible polymer mixtures based on hydrogen bonding. This is result in interesting and enhanced polymeric properties^[12,13]. The use of hydrogen bonds offers the advantage that they involve distinct donor and acceptor sites, and are very directional. In that way they offer more possibilities for structural design than forces that are symmetric and non-directional as e.g. ion-ion interactions.

In our case, we will improve Tg of polystyrene based on secondary interactions. Knowledgably crosslinking can increase Tg of polymers. However, nobody tried to use the method because few crosslinkage in polystyrene will lead to worse rheological property.

The arrangement of hydrogen bond donors and acceptors of maleimide, which are involved in hydrogen bonding in the complex with melamine, suggest that it should be possible to complex one melamine molecule to three imides units.

Fig 1-7 Proposed Structure of Maleimide/styrene interactions with melamine

Fig 1-8 Proposed Structure of Maleimide/styrene interactions with 2,6 diaminopyridine

In general, the binding strength of multiple hydrogen bonded complexes is depending on the strength of the individual hydrogen bonds in the array, and the number of hydrogen bond. Different hydrogen Donor(D) and hydrogen acceptor (A) arrays can be obtained e.g. a triple bond( DDD-AAA array, DDA-AAD array, DAD-ADA array)[12]. The DAD-ADA triple hydrogen bond array is frequently used in organic chemistry due to its synthetic availability.

The use of a hydrogen bonding unit possessing two or more interaction sites should result in network formation. It is a new, simple, economical method to prepare polystyrene with higher Tg based on hydrogen bonding crosslinkage between melamine and imide in the polystyrene prepared by free radical copolymerization of styrene and few amount of maleimide. The thermoreversible properties of hydrogen bond make the polystyrene have good rheological properties with higher Tg.

In order to reveal the interaction between imide and melamine, we used 2,6-diaminopyridine to replace melamine to complex with imide from SMA. Both of them should complex with imide by DAD-ADA arrays shown in the Fig.1-7 and Fig.1-8.

Chapter 2 Experimental

2.1 Materials

Styrene was distilled under vacuum and stored in a freezer until use. Maleimide 2,6-diaminopyridine were used commercially. AIBN (2,2'-azobutironitrile) was recrystallised from methanol before use. DMSO was purified and dried by standard techniques before use. CH₃OH and CH₂Cl₂ were used commercially.

2.2 Instruments

¹H-NMR of copolymer samples were taken in CDCl₃ on a Bruker 600 MHz spectrophotometer with DMSO as a solvent. The glass temperature transition was determined by differential scanning calorimeter (DSC) and was performed on a Perkin Elmer Pyris 1 under nitrogen with a scan rate of 10^o C/min. The glass temperature transition Tg is the midpoint in the heat-capacity change.

2.3 Synthesis of random copolymers of styrene and maleimide

Copolymer of styrene and maleimide were prepared by a free radical copolymerization using AIBN as the radical initiator and DMSO as solvent. The synthesis of random copolymers of styrene and maleimide (Fig. 2-1) which can serve as an example was performed as follow:

Styrene and Maleimide solution was prepared in DMSO. Styrene solution was added with AIBN (80% w/w of the total monomer concentration) in a 3-necked round-bottomed flask equipped with a reflux condenser, mechanical stirrer and nitrogen inlet (septum and a long needle). Maleimide solution was added dropwise with AIBN (20% w/w of the total monomer concentration) shortly and slowly. The polymer solution was precipitated in water and the resulting polymer was dissolve in CH₂Cl₂and precipitated in CH₃OH, then dried in vacuum at 60^oC.

2.4 preparation of Blends of Styrene/Maleimide copolymer and melamine or 2,6-diaminopyridine

Co-precipitation method was used to blend melamine or 2,6-diaminopyridine with imides containing copolymers. A series of experiments was done to blend Styrene-Maleimide copolymers with melamine or 2,6-diaminopyridine. Copolymers based on styrene-maleimide were mixed homogenously with various amounts of melamine and carefully dissolved in DMSO for 30 minutes, then co-precipitated in water. The resulting polymer was dissolve in CH₂Cl₂ and precipitated again in CH₃OH then dried in vacuum at 50^oC. After filtration, washing and drying the yield of the blend was determined^[7].

Chapter 3 Results and discussion

3.1 Characterization of copolymer of styrene and maleimide

3.1.1 1H-NMR Spectra

In order to characterize the random styrene-maleimide copolymer, ¹H-NMR measurement was performed on a Bruker 600 MHz spectrophotometer with DMSO as a solvent.

A typical ¹H-NMR spectrum of copolymer Maleimide Styrene is shown in figure 3-1.

Fig 3-1 ¹H-NMR spectrum of random copolymer Styrene-Maleimide at ratio 18.8/1

The ¹H-NMR spectrum shows the expected resonance for the aromatic protons of polystyrene (c and e; ä~6.0-7.6 ppm proton signals of the -CH= group of Styrene ) and imine protons of Maleimide (b; ä~ 11.2ppm).

The broad signals from 1 to 3 ppm are assignable to CH (a and f) and CH ₂(d) protons of the main chain. Therefore, this spectrum confirms the presence of Maleimide-styrene copolymer ^[15].

All of the ¹H-NMR spectra of random copolymer of Styrene-Maleimide at different compositions have shown the same characteristics bands.

3.1.2 Copolymer composition

¹H NMR spectroscopic analysis has been established as a powerful tool for the determination of copolymer compositions because of its simplicity, rapidity and sensitivity. The average composition ratio of the copolymer samples was determined from the corresponding ¹H NMR spectra. The assignment of the resonance peaks in the ¹H NMR spectrum leads to the accurate evaluation of the content of each kind of monomeric unit incorporated into the copolymer chains.

Thus, the Copolymer ratio of SMA was calculated by measuring the integrated peak areas of aromatic protons of styrene unit (signal c and e) and imine proton of Maleimide (signal b). Let B be the integrated peak areas of aromatic protons of styrene monomer and A is the integrated peak areas of imine proton of Maleimide. The polystyrene unit contains five aromatic protons active in resonance and the maleimide unit contains one imine proton, the following expression is used to determine the composition of copolymer:

Table 3-1 Calculation of integrated peak area of protons and mole compositions of copolymers I-V

Copolymer	I_Aromatic	I_Imide	A	B	[Styr]/ [Mal]
I	28.832 14.843	1.000	1.000	43.675	8.7
II	1.000 0.5473	0.0233	0.0233	1.5473	13.2
III	1.000 0.5741	0.0167	0.0167	1.5741	18.8
IV	1.000 0.5581	0.0122	0.0152	1.5581	25.5
IV	1.000 0.3540	0.0153	0.0153	1.3540	17.7

I _Aromatic Integrated peak area of aromatic protons of styrene unit.
I_mide Integrated peak area of an imine proton of Maleimide.

3.2 Copolymerization of styrene and maleimide

The random copolymer styrene-maleimide was easily obtained by radical polymerization in DMSO using AIBN as the initiator. To control the content of maleimide in copolymer, we performed the synthesis at different conditions to copolymerize Maleimide and Styrene, and obtained copolymers with identical main chain structure but with a different yield from 37.3 % to 89%. As we mentioned before, Maleimide and Styrene could constitute the charge transfer complex, so we chose dropwise addition of maleimide during the polymerization.

Experience	[Styr]/[Mal]	Reaction time/h	Temp/^oC	Masse AIBN/%	Monomer Conc./M	Yield/%	[st]/[imide]
1	10/1	6	80	5	0.0476	39.4	8.7/1
2	15/1					44.5	13.2/1
3	20/1					80.0	18.8/1
13	30/1					89.0	25.5/1

Experience	[Styr]/[Mal]	Reaction time/h	Temp/^oC	Masse AIBN/%	Monomer Conc./M	Yield /%
4	20/1	6	80	1	0.0476	37.3
5				2		48.8
6				4		59.8
3				5		80.0

Experience	[Styr]/[Mal]	Reaction time/h	Temp/oC	Masse AIBN/%	Monomer Conc./M	Yield /%
7	20/1	6	60	5	0.0476	20.8
8			70			40.0
3			80			80.0
9			90			85.0

Experience	[Styr]/[Mal]	Reaction time/h	Temp/oC	Masse AIBN/%	Monomer Conc./M	Yield /%
10	20/1	4	80	5	0.0476	67.0
11		5				72.0
3		6				80.0
12		8				81.0

From these results we conclude that the ratio of the copolymerization affects much more the yield.

Table 3-2 show that the yield of the copolymer increase with the ratio of [Styrene]/[Maleimide]. That indicates maleimide introduction slows the polymerization rate of styrene. However, the ratio of [St]/[imide] in the copolymer is less than their monomer ratio counterpart, which means Maleimide is easy to copolymerize with styrene.

Table 3-5 indicates that time is a parameter very important in the copolymerization. As we described above, maleimide was added to the solution shortly and slowly to avoid the formation of alternative copolymers and to lead predominantly to the formation of random copolymer. To extend the time reaction favors the formation of random copolymer. There is no improvement in yield at reaction time of over 6hrs. that indicates effect of maleimide on copolymerization is negligible. Table 3-3 and 3-4 have shown respectively an increase of yield when the initiator and the temperature are changed drastically. This result is demonstrated by the fact that the copolymerization reaction is depending of the amount of initiator used and the temperature required to activate the copolymerization.

3.3 Blends of the styrene/maleimide copolymers and melamine

DSC curves of the blends of the copolymers I-IV(as shown in Table 3-1) with melamine are shown in Fig 3-2 and their Tg values are presented in Table3-6.

Experience	[imide]/[Mela]	[Styr]/ [Mal]	Tg/^oC
F	1/1	18.8/1	103.3
G	1/2		113.8
H	1/3		122.0
I	1/4		125.6 105.5
J	1/5		128.0 78.0
K	1/10		130.0 90.0

Fig 3-2 DSC traces of blends with different molar concentration ratio melamine to imide in the copolymer with maleimide molar concentration of 5.05 %

As shown in Fig3-2 and in Fig3-3, addition of melamine results in a dramatic increase of Tg up to melamine concentration ratio of 3 times to imide in the copolymer which correspond to Tg equal 122^oC. In this case, Tg is 30^oC higher than the pure copolymer. At the melamine:imide ratio of 10:1, even 40^oC is reached with Tg of 130^oC. However the more addition of melamine results in presence of two value of Tg from the melamine: imide ratio of 4:1 to of 10:1. It well known there is triple-hydrogen bonding between melamine and imide unit. Recently, Ronald F. M [5,7,12] proposed that one melamine molecule interacts with three imide units, leading to a three-dimensional hydrogen bonded network.

Figure 3-3 Dependence of Tg on the ratio of melamine to imide in the copolymer

Tg increase of our copolymer in presence of melamine attributes to this kind of crosslinkage restricting the motion of polystyrene segments. The more crosslinkage in the blend corresponds to the higher Tg of the blend. However, Maleimide contains in our copolymers is 20 times less than Ronald's, and randomly distribute along the chain, which restricts the imides together to interact with melamine. Therefore, more melamine is needed to build a crosslinking site. As Figure 3-3 shown, melamine:imide ratio elevation from 3:1 to 10:1 still increases the Tg although slowly, suggesting crosslinkage density still increases. From the melamine: imide ratio of 4:1 to of 10:1, the presence of another Tg at lower temperature in case of melamine: imide ratio of 4/1, 5/1 and 10/1 is reasonably explained as the presence of free melamine which acts as a plasticizer.

In order to increase crosslinkage sites, we prepared a series of blends with different styrene/maleimide ratios. The results were given in Table 3-8 and Fig 3-4.

Experience	[Styr]/ [Mal]	Tg/^oC
A	polystyrene	100.1
B	8.1/1	84.6
C	13.2/1	87.4
D	18.8/1	91.7
E	25.5/1	93.0

Experience	[Styr]/ [imide]	[imide]/[Mela]	Tg/^oC
L	8.1/1	1/5	106.9
M	13.2/1		123.3
N	18.8/1		128.0 78.0
O	25.5/1		146.2 89.0
P	8.1/1	1/3	109.0
Q	13.2/1		113.0
H	18.8/1		122.0
R	25.5/1		127.0 74.0

Figure 3-4 reveals that Tg of copolymer decreased in a linear function with maleimide content due to the flexibility of maleimide units. However, Tg of all blends with melamine: imide ratio of 3:1 is much higher, at least 25C, than a correspondent copolymer, and is an exponential decay relationship with imide content. Blend of copolymer with the fewest imide content([styrene]/[imide] 25.5/1) has the highest Tg, 127^oC.

At the same melamine:imide ratio of 3:1, melamine isn't enough to saturate imide units in the blends with higher imide content. However, melamine is too much to complex with imide unit in the blend with imide content of 25.5/1. It is confirmed by appearing another Tg at 74^oC. So, Tg of blends with higher imide content must be much higher if more melamine is used. With melamine: imide ratio of 5:1, we prepared a series of blends and tested their Tg, shown in Table 3-8. their Tgs are much higher than ones of blends with melamine: imide ratio of 3:1.

Moreover, in the procedure of preparation of blends, it is observed that blends are readily soluble in DMSO, even in CH₂Cl₂, this indicate that crosslinling are present in our blends and is based on secondary interactions.

3.4 Blends of the copolymer and 2,6-diaminopyridine

In order to evaluate the effect of crosslinking of melamine on the Tg of blend, we prepared a series of blend of Diaminopyridine(DAP) and Styrene/Maleimide copolymer with different styrene/maleimide ratios. DAP is analogue structure to melamine. They both can form a complex with imide through a triple hydrogen bonds in the manner of DAD-ADA arrays, however without crosslinkage is formed in the blends of DAP and Styrene/Maleimide copolymer owing to difference of DAP and melamine in structure, as showed in Fig. 1-7 and Fig.1-8.

DSC curves of the blends of Diaminopyridine and Styrene/Maleimide copolymer are shown in Figure 3-5 and their T_g values are presented in Table3-9.

Experience	[imide]/[Dap]	[Styr]/ [imide]	Tg/^oC
control	1/0	17.7/1	94.2
S	1/1		95.7
T	1/2		103.9
U	1/3		105.2
V	1/4		113.0
W	1/5		115.2 78.1
X	1/10		117.2 76.5

The dependence of Tg on the ratio of DAP to imide showed that Tgs of blends are increasing with addition of DAP, as same as blends of the copolymer with melamine. Single Tg is shown till DAP concentration ratio of 4 times to imide, which correspond to Tg equal 113^oC . Then, two Tg is found although one is still elevated. the behavior is found in blends of the copolymer with melamine.

It is known there is triple-hydrogen bonding between DAP and imide unit. Tg increase of our copolymer in presence of DAP attributes to this kind of triple-hydrogen bonding existing in the blend and restricting the motion of polystyrene segments, making the copolymer look like a copolymer of styrene and complex of maleimide with DAP.

However, the Tg increase of the blends is lower ones of blends of the copolymer with melamine. Contrary to the melamine, DAP molecule can not have crosslinkng with imide unit. So the results confirm there are truly crosslinking formation in blends of the copolymer with melamine.

Chapter 4 Conclusion

Tg of polystyrene, a common polymer, is dramatically elevated based on crosslinkage between introduced imide units and melamine. Polystyrene with imide molar concentration of 5.05% has Tg of 122^oC in the presence of melamine, 22^oC higher than polystyrene, which will extend application of polystyrene. At high ratio of melamine to imide, two Tgs are observed, one is higher than 122^oC but another much lower. The existence of two Tg is due to the fact that free melamine is acting as plasticizer.

Diaminopyridine also increase Tg of polystyrene. In this case, the blend looks like a copolymer of styrene with a big monomer, complex of maleimide and Diaminopyridine. but increment is less than melamine owing to lack of crosslinking between them.

The secondary interactions between melamine or Diaminopyridine with polystyrene containing imide is confirmed by their blends readily soluble in DMSO and in CH₂Cl₂.

References Notes

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[2]	Victor Morais,Random alternate and diblock copolymers,Departement Materials,University Comlutense,Madrir,Spain(2005)
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[5]	Kumar A, Galaev IY, Mattiasson B. Affinity precipitation of á-amylase inhibiter from wheat metal by metal chelate affinity binding using Cu (\u8545centsò) loaded copolymers of 1-vinylimidazole with N-isopropy- acrylamide[J]. Biotechnol. Bioeng., 1998, 59: 693-704
[6]	Ronald lange and al.;Supramolecular Polymer Interaction based on the alternating copolymer of styrene and maleimide,laboratory of organic chemistry ,Eindhoven university,The Netherlands(1994); Supramolecular Polymer Interaction using melamine, Laboratory of organic chemistry , Eindhoven university,The Netherlands(1996)
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[10]	Bill meyer,;J.R.;Textbook of Polymer Science 3^rdEd.;Wiley interscience,New york,USA(1984)
[11]	Melamine as a dietary nitrogen source for ruminants", G.L.Newton and P.R.Utley, Journal of Animal Science, vol.47, p1338-44, 1978
[12]	E.E. Simanek, X. Li, I.S. Choi, G.M. Whitesides, "Cyanuric Acid and Melamine: A Platform for the Construction of Soluble Aggregates and Crystalline Materials", Comprehensive supramolecular chemistry, J.L. Atwood ed., New York:Pergamon, Vol.9, 495 (1996).
[13]	Ronald lange and al.;Supramolecular polymer chemistry based on melamine and maleimide-styrene copolymers,laboratory of organic chemistry Eindhoven university,The Netherlands(1997)
[14]	Coleman ,Specific interaction and the miscibility of polymer Blends ,Technomic,Lancaster(1991)
[15]	Koji Ishizu,Chisato Takashi?????Takeshi Shibuya and Satoshi Unichida, Dept. of organic materilas and Macr.,Int. research Centre of Macro.scienceTokyo Inst. of Tech.,Tokyo,Japan (2003)

Preparation of Polystyrene with Higher Tg Based on Triple Hydrogen Bond Interactions

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