Sulfide and sulfoxide based poly(ether-amide)s: Synthesis and characterization

Two new diacid monomers, 2,2′-sulfide bis(4-methyl phenoxy acetic acid) and 2,2′-sulfoxide bis(4-methyl phenoxy acetic acid) were successfully synthesized by refluxing the 2,2′-sulfide bis(4-methyl phenol) and 2,2′-sulfoxide bis(4-methyl phenol) with chloroacetonitrile in the presence of potassium carbonate, and subsequent basic reduction. Two novel series of poly(sulfide-ether-amide)s and poly(sulfoxide-ether-amide)s with aliphatic units in the main chain were prepared from diacids with various diamines.The polyamides were obtained in quantitative yields and their inherent viscosities were in the range of 0.43-0.89 dl g⁻¹ at a concentration of 0.5 g dl⁻¹ in N,N-dimethylacetamide (DMAc) solvent at 25 °C. They showed good thermal stability. The temperature for 10% weight loss in argon atmosphere was in the range of 350-415 °C. The polymers showed glass transition temperatures between 228 and 261 °C. Almost all of the polyamides were readily soluble in a variety of polar solvents such as N-methyl-2-pyrrolidone (NMP) and dimethyl sulfoxide (DMSO).     

Examining thermal stability and structure property relationships in coatings based on linear aromatic poly(methoxy-thiocyanurate)s

A series of copoly(methoxy-thiocyanurate)s is prepared in good yield and purity, and fully characterised. Many of the resulting polymers, formed at room temperature using phase transfer catalysis, can be cast into films with good resilience and thermal stability (some examples suffer practically no mass loss when held isothermally at 190 °C and only display appreciable losses when held continuously at 225 °C). Char yields of 61–64% are achieved in nitrogen depending on backbone structure. Some problems were encountered with solubility, particularly with copolymers, which limited molecular weights analysis, but values of M_n = 7000–10,000 g mol⁻¹ were obtained for the polycyanurate and polythiocyanurate homopolymers. DSC reveals polymerisation exotherms with maxima at 197–207 °C (ΔH_p = 39–48 kJ/mol), which are believed to be due to isomerisation of the (activation energies span 172–205 kJ/mol), since X-ray powder diffraction measurements reveal no evidence of crystalline structure in the resulting product.     

Synthesis, characterization and thermal degradation mechanism of three poly(alkylene adipate)s: Comparative study

Three high molecular weight aliphatic polyesters derived from adipic acid and the appropriate diol - poly(ethylene adipate) (PEAd), poly(propylene adipate) (PPAd) and poly(butylene adipate) (PBAd) - were prepared by two-stage melt polycondensation method (esterification and polycondensation) in a glass batch reactor. Intrinsic viscosities, GPC, DSC, NMR and carboxylic end-group measurements were used for their characterization. Mechanical properties of the prepared polyesters showed that PPAd has similar tensile strength to low-density polyethylene while PEAd and PBAd are much higher. From TGA analysis it was found that PEAd and PPAd have lower thermal stability than poly(butylene adipate) (PBAd). The decomposition kinetic parameters of all polyesters were calculated while the activation energies were estimated using the Ozawa, Flynn and Wall (OFW) and Friedman methods. Thermal degradation of PEAd was found to be satisfactorily described by one mechanism, with activation energy 153 kJ/mol, while that of PPAd and PBAd by two mechanisms having different activation energies: the first corresponding to a small mass loss with activation energies 121 and 185 kJ/mol for PPAd and PBAd, respectively, while the second is attributed to the main decomposition mechanism, where substantial mass loss takes place, with activation energies 157 and 217 kJ/mol, respectively.     

Novel processable and heat resistant poly(phenylthiourea azomethine imide)s: Synthesis and characterization

N-(4-chloro-3-aminobenzal)N′(4-aminophenyl)thiourea having phenylthiourea and azomethine groups was synthesized and exploited as starting material for the fabrication of new polymers. Novel diamine was condensed with pyromellitic dianhydride, 3,3′,4,4′-benzophenone-tetrcarboxylic dianhydride and 4,4′-(hexafluoroisopropylidene) diphthalic dianhydride to obtain poly(phenylthiourea azomethine imide)s. The structural explication of monomers and poly(phenylthiourea azomethine imide)s were carried out by FTIR, ¹H and ¹³C NMR techniques along with crystallinity, organosolubility, inherent viscosity and molecular weight measurements. Accordingly, polymers bearing CS and -CN- moieties in the backbone demonstrated an amorphous nature and were readily soluble in amide solvents such as DMAc, DMF, and DMSO. Poly(phenylthiourea azomethine imide)s encompassed η_inh of 1.40-1.55 dL/g and were obtained in quantitative yields. In addition, GPC measurements of polymers revealed M_w around 60,291-67,665. Thermal stability of these polymers was ascertained via 10% weight loss temperatures around 514-533 °C in an inert atmosphere. Besides, glass transition temperatures of polyimides were found to be 272-276 °C.     

Synthesis and characterization of new telechelic poly(phenyleneoxide)s

New well-defined telechelic poly(phenyleneoxide)s (PPO's) were synthesized from 4-bromo-2,6-dimethylphenol and bi-phenolic compounds through phase transfer catalyzed aromatic nucleophilic substitution polymerization. Bisphenol-A (BPA), 4,4^′-biphenol (BP), hydroquinone (HQ) and 2,6-dihydroxynaphthalene (DHN) were employed as telechelic units. The composition analysis by proton-nuclear magnetic resonance (¹H-NMR) spectroscopy revealed that DHN was highly reactive compared to BPA and HQ, whereas BP was un-reactive in the polymerization process. The number average repeating unit (n) in telechelic PPO was estimated as n=17-19 and n=17-20 for DHN and BPA (or HQ), respectively. The reactivity of the bi-phenolic in PPO synthesis are confirmed as DHN > HQ ∼ BPA ? BP. The molecular weight determination by gel permeation chromatography (GPC) and viscosity method suggest that the molecular weight of PPO decreased drastically with increasing amount of bi-phenolic units in the feed. The GPC chromatogram of PPO showed a bi-modal distribution, clearly indicative of formation of two different types of molecular weight chains, whereas the telechelic polymers have a mono-modal distribution with a narrow polydispersity. Thermal analysis by differential scanning calorimetry revealed that telechelic polymers are highly amorphous, like PPO, and no crystallization or melting peaks were observed in the heating/cooling cycles.     

Synthesis and characterization of poly(vinylpyrrolidone-co-vinylphosphonic acid) copolymers

Radical copolymerizations of 1-vinyl-2-pyrrolidone (VP) with vinylphosphonic acid (VPA) at different feed ratios were investigated. The copolymers were characterized by ¹H-NMR, ¹³C-NMR and FT-IR. The copolymer composition was determined from the elemental analysis. Thermogravimetric analysis (TG) illustrates that the copolymers are stable up to 200 °C. Temperature dependence of the alternating current (AC) conductivities were investigated by means of impedance spectroscopy. The direct current (DC) conductivities of the samples are derived from the AC conductivity data.     

Synthesis and characterization of new poly(ester-amide)s containing diarylidenecyclohexanone in the main chain. Part: II

A new category of poly(ester-amide)s based on diarylidenecyclohexanone in the main chain were synthesized via interfacial polycondensation of two monomers namely: 2,6-bis(4-hydroxybenzylidene) cyclohexanone I and 2,6-bis(4-hydroxy-3-methoxybenzylidene) cyclohexanone II with diacid chlorides IIIa–c. The model compounds were synthesized by reacting one mole of compounds IVa–c with two monomers I and II. The structure of the model compounds was confirmed by correct elemental and spectral analyses. The various characteristics of the resulting polymers including: solubility, viscosity, thermal analysis, and X-ray diffraction analysis were determined and discussed. The majority of the polymers were insoluble in most common organic solvents. The viscosity measurements in dimethylsulfoxide showed the values 0.58–0.79 dl/g. Thermal analysis showed that they were thermally stable up to 500 °C. X-ray analysis showed that the polymers had some degree of crystallinity in the region 2θ = 5–50°.     

Preparation,characterization and thermal degradation study of poly(amide imide)s based on tri-component mixture of PMDA/BTDA,diamines and acid chloride

A series of poly(amide imide)s (PAIs) having alternate (amide–amide) and (imide–imide) units (polymers 1–14 and 22–35), and random distribution of amide-imide linkages (polymers 15–21 and 36–42) were prepared by low temperature solution polymerization of benzene-1,2,4,5-tetracarboxylic dianhydride (PMDA)/benzophenone-3,3′,4,4′-tetracarboxylic dianhydride (BTDA), diamines (cyclic and aromatic) and acid chloride in dimethylforamide. All the polymers were readily soluble in polar aprotic solvents with inherent viscosities in the range of 0.134–0.878. The process of cycloimidization of poly(amide amic acid)s (PAAs) to PAIs was investigated by TGA and FT-IR techniques at four different temperatures i.e., 175, 200, 225, and 260 °C. The rate of cycloimidization was calculated by taking into account the theoretical weight loss (W_T), obtained from [n × M_w (H₂O)/M_w (R_U)] W, where M_w (H₂O) molecular weight of water, W weight of PAA taken for TGA, M_w (R_U) the molecular weight of repeat unit of PAA, n number of water molecules eliminated per repeat unit of PAA upon cycloimidization. For a particular diamine, the extent of percentage cycloimidization at the end of the isothermal heating was higher for PAAs containing trimellitic anhydride chloride (TMAc) unit, irrespective of the nature of the dianhydride and diamine. Thermal and thermooxidative degradation of PAIs was investigated by TGA in nitrogen and oxygen atmosphere. The initial decomposition temperatures (IDT) of polymers are above 260 °C, and vary widely (from 260 to 501 °C) depending upon the structure of the polymer backbone. PAIs containing TMAc exhibited higher thermal stability as compared to those polymers having diacid chloride units, in both N₂/O₂ atmospheres.     

Synthesis and characterization of carborane-containing poly(hydroxy ethers) with excellent thermal stability

o-Carborane-containing poly(hydroxy ethers)(P1, P2 and P3) were synthesized via "advancement reaction" of o-carborane-containing bisphenol(4) and diglycidyl ether of bisphenols(DGEBA and 1). FTIR and ~(1) H-, ~(13)C-, and ~(11) B-NMR were utilized to characterize the obtained polymers. TGA test was conducted under nitrogen and air. It is found that the shielding effect of carborane moiety on its adjacent aromatic structures contributes to high initial decomposition temperatures, while oxygen in air has an adverse effect on the initial decomposition temperature. The oxygen can combine with polymer chain to form peroxide and hydroperoxide groups, which are more reactive during the degradation process. Besides, o-carborane-containing poly(hydroxy ethers) have high char yield at elevated temperatures. The boron atom combines with oxygen from the polymer structure or/and from air, thus to form a three-dimensional network linked with B―O―B and B―C bonds, and retain the polymer weight to a large extent.     

Synthesis and characterization of novel poly（aryl ether）s containing naphthyl moieties

Two poly(aryl ether)s containing naphthyl moieties were prepared from bis(3,5-dimethyl-4-hydroxyphenyl)naphthyl methane (monomer 1) via nucleophilic aromatic substitution polycondensation with bis(4-fluorophenyl) ketone and bis(4-fluorophenyl) sulfone.The structures of these polymers were confirmed by ~1H NMR.The M_n values of the two polymers were 96,200 and 88,600, respectively.The polymers exhibited good thermal stabilities with 5%mass loss at T>400℃and high glass-transition temperature(T_g) of T>250℃...     

Synthesis and characterization of calcium containing poly(urethane-ether)s

Calcium containing poly(urethane-ether)s (PUEs) were synthesized by the reaction of hexamethylene diisocyanate or toluylene 2,4-diisocyanate (HMDI or TDI) with a mixture of calcium salt of mono(hydroxybutyl)phthalate [Ca(HBP)₂] and polyethylene glycol (PEG₂₀₀ or PEG₄₀₀). A series of calcium containing PUEs having different composition were synthesized by taking the mole ratio of Ca(HBP)₂:PEG₂₀₀ or PEG₄₀₀:diisocyanate (HMDI or TDI) as 3:1:4, 2:2:4 and 1:3:4 to study the effect of calcium content on the properties of the copolymer. The structure of the polymers were confirmed by IR, ¹H-NMR, ¹³C-NMR, and solid state ¹³C-CP-MAS NMR. The polymers were soluble in dimethyl sulfoxide and dimethyl formamide. The initial decomposition temperature of the polymers decreases with increase in calcium content. The T_g value of PUEs increases with increase in calcium content and decreases with increase in soft segment content and length. A single T_g value is observed for the calcium containing PUEs based on PEG₂₀₀ shows the presence of homogeneous phase. However, two T_g values for the PUEs based on PEG₄₀₀ for various composition of Ca(HBP)₂, PEG₄₀₀ and diisocyanate (HMDI or TDI) shows the presence of heterogeneous phase. The viscosity of the calcium containing PUEs increases with increase in the soft segment content as well as its length and decreases with increase in calcium content. X-ray diffraction patterns of the polymers show that the HMDI based polymers are partially crystalline and TDI based polymers are amorphous in nature. The dynamic mechanical analysis of the calcium containing PUEs based on HMDI shows that at any given temperature modulus (g^′ and g^″) increases with increase in the ionic content in the polymers.     

Synthesis,characterization and quantum yields of multichromic poly(azomethine)s containing carbazole unit

Poly(azomethine)s containing phosphor, silane and carbazole were synthesized with multiple stage and examined through different photophysical, electrochemical, and thermal behaviours. Following substances were synthesized as an initial step: N-hexyl-carbazole (CH) and N-hexyl-carbazolaldehyde (CHDA), via elimination reaction in argon media and N-hexyl-carbazole azomethine (CHA) via condensation reaction of CHDA with 4-aminophenol also poly(azomethine)s (P-Si-CHA, P-P-CHA) containing silane and phosphor via elimination reaction. The structures of synthesized compounds were confirmed by Fourier-transform infrared spectroscopy (FT-IR), Ultraviolet–visible spectroscopy (UV–Vis) and nuclear magnetic resonance spectroscopy (NMR). Electrochemical properties of compounds were examined with cyclic voltammetry (CV) technique. Fluorescence measurements were utilized to investigate the photochemical behaviors by photoluminescence (PL) analysis. CHA compound surprisingly presented multicolor emission behavior (when excited at 370, 420, 480, 540 and 580 nm, the solution emitted blue, green, yellow, orange and red lights, respectively) with relatively high quantum yield (19.9%) in DMF solvent. Additionally, thermal behaviors of all compounds were determined by TG and DSC techniques. Surface morphologies of polymers were imaged with scanning electron microscope (SEM).     

Synthesis and characterization of poly(amideimide)s containing pendent flexible alkoxy chains

A series of aromatic diacylhydrazides containing pendent flexible alkoxy chains, viz., 5-butyloxyisophthalicacid dihydrazide, 5-octyloxyisophthalicacid dihydrazide, 5-dodecyloxyisophthalicacid dihydrazide and 5-hexadecyloxyisophthalicacid dihydrazide were synthesized by the hydrazinolysis reaction of the corresponding aromatic esters with hydrazine hydrate. Diacylhydrazides were each polycondensed with aromatic dianhydrides, viz., 4,4′-oxydiphthalic anhydride (ODPA), 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA) and pyromellitic dianhydride (PMDA) to obtain new poly(amideimide)s. Poly(amideimide)s had inherent viscosity in the range 0.55-0.88 dL/g in N,N-dimethylacetamide (DMAc) at 30 ± 0.1 °C. Poly(amideimide)s were found to be soluble in DMAc, N,N-dimethylformamide (DMF), 1-methyl-2-pyrrolidinone (NMP) and pyridine and could be cast into tough, flexible and transparent films from a solution in DMAc. X-ray diffractograms revealed that poly(amideimide)s with longer alkoxy chains had layered structures. Glass transition temperature of poly(amideimide)s containing pendent flexible alkoxy chains were in the range 215-245 °C. Temperature at 10% weight loss was in the range 380-410 °C in nitrogen atmosphere indicating good thermal stability of poly(amideimide)s.     

Synthesis and properties of novel aromatic poly(ester-imide)s bearing 1,5-bis(benzoyloxy)naphthalene units

Two series of new aromatic poly(ester-imide)s were prepared from 1,5-bis(4-aminobenzoyloxy)naphthalene (p-1) and 1,5-bis(3-aminobenzoyloxy)naphthalene (m-1), respectively, with six commercially available aromatic tetracarboxylic dianhydrides via a conventional two-stage synthesis that included ring-opening polyaddition to give poly(amic acid)s followed by chemical imidization to polyimides. The intermediate poly(amic acid)s obtained in the first stage had inherent viscosities of 0.41-0.84 and 0.66-1.37 dl/g, respectively. All the para-series and most of the meta-series poly(ester-imide)s were semicrystalline and showed less solubility. Two of the meta-series poly(ester-imide)s derived from less rigid dianhydrides were amorphous and readily soluble in polar aprotic solvents, and they could be solution-cast into transparent and tough films with good mechanical properties. The meta-series polymers derived from rigid dianhydrides were generally semicrystalline and showed less solubility. Except for one example, the meta-series poly(ester-imide)s displayed discernible T_gs in the range 239-273 °C by DSC. All of these two series poly(ester-imide)s did not show significant decomposition below 450 °C in nitrogen or in air.     

Synthesis, photoluminescent and electrochromic properties of new aromatic poly(amine-hydrazide)s and poly(amine-1,3,4-oxadiazole)s derived from 4,4′-dicarboxy-4″-methyltriphenylamine

A series of new poly(amine-hydrazide)s I were prepared from the dicarboxylic acid 4,4′-dicarboxy-4″-methyltriphenylamine with terephthalic dihydrazide (TPH) and isophthalic dihydrazide (IPH), respectively, via the Yamazaki phosphorylation reaction. Polymers I were readily soluble in many common organic solvents, and could be solution cast into transparent, tough, and flexible films with good mechanical properties. Differential scanning calorimetry (DSC) indicated that the hydrazide polymers had T_g’s in the range of 222-223 °C and could be thermally cyclodehydrated into the corresponding oxadiazole polymers in the range of 300-400 °C. The resulting poly(amine-1,3,4-oxadiazole)s II exhibited T_g’s in the range of 269-283 °C, 10% weight-loss temperatures in excess of 511 °C, and char yield at 800 °C in nitrogen higher than 63%. These poly(amine-hydrazide)s I exhibited strong UV-Vis absorption bands at 351-355 nm in NMP solution. Their photoluminescence spectra in NMP solution and film showed maximum bands around 459-461 nm in the blue region for I series. The hole-transporting and electrochromic properties are examined by electrochemical and spectroelectrochemical methods. Cyclic voltammograms of the poly(amine-hydrazide)s I prepared by casting polymer solution onto an indium-tin oxide (ITO)-coated glass substrate exhibited one reversible oxidation redox couples at 1.32-1.33 V vs. Ag/AgCl in acetonitrile solution. All obtained poly(amine-hydrazide)s I revealed excellent stability of electrochromic characteristics, changing color from original pale yellowish to blue.     

Hyperbranched aromatic poly(ether imide)s: Synthesis,characterization, and modification

The synthesis and characterization of hyperbranched aromatic poly(ether imide)s are described. An AB₂ monomer, which contained a pair of phenolic groups and an aryl fluoro moiety activated toward displacement by the attached imide heterocyclic ring, was prepared. The nucleophilic substitution of the fluoride with the phenolate groups led to the formation of an ether linkage and, subsequently, to the hyperbranched poly(ether imide), which contained terminal phenolic groups. A similar one‐step polymerization involving a monomer that contained silyl‐protected phenols yielded a hyperbranched poly(ether imide) with terminal silylated phenols. The degree of branching of these hyperbranched polymers was approximately 55%, as determined by a combination of model compound studies and ¹H NMR integration experiments. End‐capping reactions of the terminal phenolic groups were readily accomplished with a variety of acid chlorides and acid anhydrides. The nature of the chain‐end groups significantly influenced physical properties, such as the glass‐transition temperature and the solubility of the hyperbranched poly(ether imide)s. As the length of the acyl chain of the terminal ester groups increased, the glass‐transition temperature value for the polymer decreased, and the solubility of the polymer in polar solvents was reduced, becoming more soluble in nonpolar solvents. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 2536–2546, 2001     

Regularly alternating poly(amideimide)s containing pendent pentadecyl chains: Synthesis and characterization

Two new aromatic diamines containing preformed amide linkages, viz., N,N′-(4-pentadecyl-1,3-phenylene)bis(4-aminobenzamide) I and N,N′-(4-pentadecyl-1,3-phenylene)bis(3-aminobenzamide) II, were synthesized by reaction of 4-pentadecylbenzene-1,3-diamine with 4-nitrobenzoylchloride and 3-nitrobenzoylchloride, followed by reduction of the respective dinitro derivatives. A series of new poly(amideimide)s was synthesized by polycondensation of I and II with four commercially available aromatic dianhydrides, viz., pyromellitic dianhydride (PMDA), 4,4′-biphenyltetracarboxylic dianhydride (BPDA), 4,4′-oxydiphthalic anhydride (ODPA), and 4,4′-(hexafluoroisopropylidene)diphthalic anhydride (6-FDA) in N,N-dimethylacetamide (DMAc) employing conventional two step method via poly(amic acid) intermediate followed by thermal imidization. Reference poly(amideimide)s were synthesized by polycondensation of N,N′-(1,3-phenylene)bis(4-aminobenzamide) and N,N′-(1,3-phenylene)bis(3-aminobenzamide) with the same aromatic dianhydrides. Inherent viscosities of poly(amideimide)s containing pendent pentadecyl chains were in the range 0.37-1.23 dL/g in N,N-dimethylacetamide at 30 ± 0.1 °C indicating the formation of medium to high molecular weight polymers. The poly(amideimide)s containing pendent pentadecyl chains were found to be soluble in N,N-dimethylacetamide, N,N-dimethylformamide, 1-methyl-2-pyrrolidinone and pyridine and could be cast into transparent, flexible and tough films from their N,N-dimethylacetamide solution. Wide angle X-ray diffraction patterns exhibited broad halo indicating that the polymers were essentially amorphous in nature. X-ray diffractograms also displayed sharp reflection in the small angle region (2θ ≈ 3°) for poly(amideimide)s containing pentadecyl chains indicating the formation of layered structure arising from packing of flexible pentadecyl chains. The glass transition temperatures observed for reference poly(amideimide)s were in the range 331-275 °C and those for poly(amideimide)s containing pendent pentadecyl chains were in the range 185-286 °C indicating a large drop in T_g owing to the “internal plasticization” effect of pentadecyl chains. The temperature at 10% weight loss (T₁₀), determined by TGA in nitrogen atmosphere, were in the range 460-480 °C indicating their good thermal stability.     

Sulfur-containing polymers. Synthesis and properties of novel poly(arylene thioether)s based on 2,2-bis(4-mercaptophenyl)propane

Novel poly(arylene thioether)s were synthesised using the one pot polymerisation method from the bis(N,N-dimethyl-S-carbamate) of 2,2-bis(4-mercaptophenyl)propane and activated difluoro aromatic compounds. Each of the polymers, obtained in good yields and with fairly high molecular weights, were characterised by IR, GPC and NMR analyses. In particular, the NMR characterisation was performed using ¹H NMR, ¹³C NMR, DEPT, 2D COSY and 2D HSQC experiments. To study the effect on the thermal properties of replacing the ethereal oxygen atoms with sulfur atoms, we have synthesised the counterparts, the poly(arylene ether)s, with similar molecular weights. We observed that the substitution of the ethereal oxygen atoms with sulfur atoms results in a slightly lower thermal stability for the poly(thioether ketone)s, both under nitrogen and in air, and does not modify the T_g values. For the poly(thioether sulfone), on the other hand, the thermal stability is equal to that of the poly(ether sulfone), while the T_g is 10 °C lower. Furthermore, each sample is completely amorphous, with the exception of one of the poly(thioether ketone)s (sample 1b), which shows the capacity to crystallise, even if with a very slow crystallisation kinetics.     

Synthesis and characterization of new poly(arylene ether)s containing heterocyclic units. II.

A series of new poly(arylene ether)s, containing naphthalene, pyridine, and quinoline units have been prepared by solution condensation polymerization. The synthesis involves nucleophilic displacement of aromatic dihalides with aromatic potassium bisphenates in an anhydrous dipolar aprotic solvent at elevated temperatures. The polymers, having inherent viscosity from 0.24 to 1.32 dL/g, were obtained in quantitative yield, have excellent thermal stability as shown by 10% weight loss temperatures in nitrogen and air (above 450 and 430°C, respectively) and high glass transition temperatures (in the range of 150–220°C). The introduction of quinoline moieties in the polymer backbone positively influences the thermal properties, such as high T_g/T_m ratios. © 1995 John Wiley & Sons, Inc.