Influence of molecular weight on dynamic crossover temperature in linear polymers

Dielectric and light scattering spectra of two linear polymers, polyisoprene (PIP) and polystyrene (PS), were analyzed in broad temperature and frequency range above the glass transition temperature, T_g. The crossover temperature, T_C, was estimated using two approaches: (i) derivative analysis of relaxation times proposed by Stickel and (ii) Mode-Coupling Theory approach. Both estimates provide consistent values. T_C varies with molecular weight (MW) in both polymers, while the ratio T_C/T_g changes significantly with MW in PS only. It appears that the segmental relaxation time at T_C has value τ(T_C) ∼ 10⁻⁷ s for both polymers independent of MW and similar to the value reported for many non-polymeric glass-forming systems. No sign of the dynamic crossover has been observed in the chain relaxation around T_C of the segmental dynamics.     

Mixing effects on the mid-kilohertz mobility of polystyrene in glassy polystyrene-diluent blends

Low molecular weight additive effects on the mid-kilohertz mobility of atactic polystyrene (PS) at 25°C via n.m.r. spin-lattice relaxation experiments in the rotating frame (T_1?) are analogous to the results of a previous study of diluents in glassy bisphenol-A polycarbonate. The diluent with a relatively low glass transition temperature (T_g), dioctylphthalate, increases the spectral density of thermal motions of the chain backbone and the pendant group in PS on the order of 38.5 kHz. Of the ¹³C nuclei in the glassy polymer, the relaxation behaviour of which can be differentiated by high-resolution, solid-state n.m.r., the T_1?'s of the aromatic carbons in the side group are affected most by dioctylphthalate. In contrast, the styrene oligomer, which has a higher T_g than that of dioctylphthalate, does not significantly alter the ambient temperature mobility of polystyrene at 38.5 kHz. The PS-styrene oligomer and PS-dioctylphthalate blends are examples of athermal and non-athermal mixtures, respectively. However, the effect of the enthalpy of mixing on the T_1?'s of the polymer is probably obscured by differences in blend mobility due to different blend T_g's.     

Dynamic mechanical properties of polystyrene–polyurethane blends

Four polystyrene–polyurethane mechanical blends were prepared with 5, 10, 20, and 40% thermoplastic polyurethane, respectively. Their impact properties were compared with pure polystyrene and commerical types of impact polystyrene. The rheological properties of the blends were studied with DSC and dynamic mechanical spectroscopy. It was found that addition of softer polyurethane conglomerates embedded inside the polystyrene matrix, although increasing the toughness of the blend as expected from addition of the softer particulate, also increased the glassy region of the blends by shifting their T_gs to higher temperatures. A theory based on the interaction of phases was propounded explaining this phenomenon.     

Blends of syndiotactic polystyrene with SEBS triblock copolymers

Blending of styrene-b-(ethylene-co-1-butene)-b-styrene (SEBS) triblock copolymers with syndiotactic polystyrene (PSsyn) has been performed in a Brabender mixer above the higher glass transition temperature of the triblock copolymer but below the PSsyn melting point. The large excess of the triblock copolymer over the homopolymer as well as the significant amount of plasticized amorphous PSsyn phase allowed the easy processing under the used temperature conditions with good interface compatibility. The consequent interfacial adhesion between the amorphous PS phase and the unmelted PSsyn crystallites affects both the final morphology of the blend as well as its dynamic behavior. Indeed, such solid particles act as reinforcing point of the overall blend structure, as evidenced by scanning electron microscopy. Moreover, they contribute to a T_g increase in the order of 20 °C with respect to pure SEBS and to an appreciable conservation of mechanical properties at temperatures higher than the T_g of the PS blocks of SEBS. The mechanical and thermal behavior of the synthesized blends has been studied and tentatively correlated to the molecular weight ratio between PSsyn and the PS blocks of SEBS.     

Effect of cavitations on brittle-ductile transition of particle toughened thermoplastics

In this study, we established a correlation between cavitations volume and the brittle-ductile transition (BDT) for particle toughened thermoplastics. The brittle-ductile transition temperature (T_BD) was calculated as a function of T^∗ and interparticle distance (ID), respectively, where T^∗ was a parameter related to the volume of cavitations. The results showed that the smaller the cavitations volume, the higher the brittle-ductile transition temperature. The calculations correlated well with the experimental data. With respect to rubber particle, the rigid particle was too hard to be voided during deformation, thereby the T_BD of the blend was much higher than that of rubber particle toughened thermoplastic. This was a main reason that rubber particle could toughen thermoplastics effectively, whereas rigid particle could not.     

Glass transition temperature of ideal polymeric networks

Measurements of the glass transition temperature (T_g) have been carried out on polystyrene networks prepared by the anionic copolymerization of styrene and divinylbenzene, and star-shaped polystyrene of varying functionality. The results show a linear variation of T_g versus $\text{M}\text{?}{}^{\mathrm{?1}}{}_{\mathrm{n}}$ in all cases. The value of the slope interpreted in terms of the free volume theory shows that the glass transition temperature depends closely on the average functionality of the crosslinks. In order to study the influence of free chains on the glass transition of crosslinked polymers a series of networks were contaminated with increasing ratios of linear polystyrene chains, slightly polydisperse.     

The adhesion of amorphous polystyrene surfaces below Tg

The bonding of polystyrene (PS) surfaces below T_g was investigated by two different fracture tests: the lap-shear joint method and the cantilever beam method. Adhesion energy values obtained by the two methods are in agreement and develop with (time)^1/2, at temperatures as low as T_g−16 °C. Even if the double cantilever method is the most common test found in the literature for adhesions above T_g, for low adhesion values, below T_g, the lap-shear joint geometry is more appropriate. Moreover, when the glass transition temperature is used as a reference temperature, polydisperse and monodisperse PS adhesion energy curves are superposable, suggesting that the auto-adhesion is not significantly favored by the presence of numerous chain ends at the surface (due to the low molecular weight chains provided by the polydisperse PS).     

Strengthening and toughening of a multi-component lithium disilicate glass-ceramic by ion-exchange

A multi-component lithium disilicate (LD) glass-ceramic with interlocking microstructure consisting of rod-like LD crystals and glassy matrix was ion-exchanged over wide temperature and time ranges in pure NaNO₃ or mixed NaNO₃ and KNO₃ baths below the glass transition temperature. Treatment temperature, time and salt bath dependences of surface characteristics and mechanical properties for the ion-exchanged glass-ceramic were investigated. It was found that the glass-ceramic with limited glassy matrix could be remarkably strengthened and toughened in NaNO₃ bath by adjusting the treatment temperature to a moderate level, at which Li⁺/Na⁺ exchange between the glassy matrix and the salt bath could form an ion-exchanged layer with larger depth and less stress relaxation. Furthermore, by using the mixed salt bath, the undesirable exchange of K⁺/Na⁺ in pure NaNO₃ bath could be limited; further enhanced strengthening effect was achieved. The results might renew the interest on strengthening LD glass-ceramics by traditional ion-exchange process.     

Empirical determination of equation of state for zero internal pressure in rare gas solids and semi-crystalline polymers

An equation of state for zero internal pressure in rare gas solids and semi-crystalline polymers has been determined based on the empirical functions of thermal pressure coefficient γ_V with respect to volume at constant pressure. The experimental data of PVT over wide range of temperature and pressure published by Anderson and Swenson and Syassen and Holzapfel for rare gas solids and Olabisi and Simha and Zoller for semi-crystalline polymers are used to evaluate γ_V. The function of γ_V with respect to volume determined at constant pressure is given by where V₀ is the volume at 0 K, A, ? and c are constants. The function of internal pressure P_i = γ_VT − P with respect to temperature at constant pressure is determined by converting the function of γ_V(V) to a function of temperature γ_V(T). An empirical equation of state for zero internal pressure determined by pressure P^∗, volume V^∗ and temperature T^∗ at which P_i = 0 is expressed by P^∗V^∗/RT^∗=C^∗−D^∗V^∗ for rare gas and semi-crystalline polymer where C^∗ and D^∗ are constants. The practical meaning of the equation of state for P_i = 0 in the semi-crystalline polymers has been discussed.     

Origin of the divergence of the timescales for volume and enthalpy recovery

Although the relationship between the relaxation timescales of thermodynamic, mechanical, viscoelastic, and dielectric properties in amorphous materials has been studied extensively, no general consensus has been reached. In this work, we examine the relationship between the timescales of volume and enthalpy relaxation for polystyrene using the cooling rate dependence of the glass transition temperature (T_g) obtained from capillary dilatometry and differential scanning calorimetry (DSC). Our analysis suggests that both volume and enthalpy exhibit similar relaxation timescales at temperatures above and below T_g. The divergence of the times required to reach equilibrium noted in the literature at temperatures several degrees below the nominal T_g is attributed to the effects of nonlinearity. The relationship between nonlinearity and dynamic heterogeneity is discussed.     

The gel-phase modulus of high-impact polystyrene from dynamic mechanical spectroscopy

High-impact polystyrene (HIPS) constitutes a mechanically attractive composite, consisting of a glassy matrix and a rubberlike particle phase (gel phase). Dynamic mechanical spectroscopy was performed for the polystyrene matrix for three different types of HIPS as well as for the concentrated gel-phase material, at the vicinity of the respective glass-transition temperatures (T_g). An approximate estimation of the gel-phase modulus was attempted by using known mechanical models. A comparison with experiments was also made. The modulus of the composite was found to be lower than the theoretical lower bound for particulate composites. This was attributed to a separate phase between gel particles and the matrix. A diffusion-type variation of the modulus of this mesophase layer was estimated, and a correlation between calculated fitting parametric exponents and impact behavior of HIPS was found. Moreover, the T_gs of the materials under investigation were also measured with two independent methods. It was found that all types of HIPS presented higher T_gs than the pure matrix by 5 to 10°C with the highest T_g found being that of the gel-enriched material. The shift of T_gs to higher temperatures was attributed to an eventual increase of the effective cross-link density of the matrix because of grafting.     

Structure–property relationships for styrene crosslinked polyesters. II. Glass transition temperature

The glass transition temperature of 10 styrene-free polyester prepolymers and the corresponding networks crosslinked by about 40% by weight of styrene were determined by DSC (and by DMA in the case of networks). The glass transition temperature T_gL of an hypothetical copolymer containing all the difunctional units of the network was then calculated. It is an increasing function of the molar weight of the prepolymer and of the phthalate/maleate molar ratio. From IR and NMR measurements, it was established that the structural irregularities other than polyester chain ends, especially unreacted double bonds, can be neglected to a first approximation. A constituent repeat unit (CRU) defined on the basis of these results allows the calculation of the crosslink density n. Then, various theories of the effect of n on T_g are compared. It appears that neither the Fox and Loshaek nor the Di Marzio approach gives good results. The crosslinking constants are lower than those found for aliphatic skeleton polyesters or epoxies. In the series under study, they display a tendency to decrease with the aromatic content. Some possible reasons of this peculiar behavior are discussed.     

Problems of compatibility of the values of glass transition temperatures published in the world literature

The influence of different factors (primarily, the temperature-time conditions for preparation and measurement of samples) on the glass transition temperatures determined from the temperature dependences of properties is analyzed using the calculations performed in terms of the relaxation theory of glass transition. The most optimum conditions for measurement of the glass transition temperatures T _g that ensure the compatibility of the values of T _g obtained by different researchers are recommended. The validity of the assertion that the glass transition temperature T _g is a temperature at which the viscosity of glasses is equal to 10¹³ P is considered.     

The effect of flexible chain length on thermal and mechanical properties of poly(m-methylene 2,6-naphthalate)s

The effect of flexible chain length on the thermal and mechanical properties such as melting temperature, glass transition temperature, dynamic mechanical relaxation behavior, and uniaxial tensile deformation for melt-quenched poly(m-methylene 2,6-naphthalate) (PmN) films was investigated using differential scanning calorimeter (DSC), dynamic mechanical thermal analyzer, and universal tensile machine. It was found from DSC thermograms that PmNs with even number of methylene group have higher melting temperatures and faster crystallization rates than PmNs with odd number of methylene group, showing an odd-even fluctuation. The plots of versus temperature show that all PmN samples have three relaxation processes (β, β^∗, and α) regardless of the number of methylene group in their backbone. It was found that both β^∗- and α-relaxations are cooperative processes and that the activation energies of both relaxations as well as the glass transition temperature associated with the α-relaxation show odd-even fluctuations as a function of the number of methylene group. The initial tensile modulus at the low drawing rate of 0.15 cm/min also shows an odd-even fluctuation. In summary, the macroscopic thermal and mechanical properties of PmN such as melting temperature, glass transition temperature, crystallization rate, activation energies of α- and β^∗-relaxations, and initial modulus measured under a slow drawing rate exhibit odd-even fluctuations as the number of methylene group in PmN increases.     

Thermally stimulated recovery of plastic strain in crosslinked and uncrosslinked epoxy/amine systems

Effects of crosslinks on plastic strain recovery in epoxy glass was studied by means of thermally stimulated strain recovery (TSSR) and differential scanning calorimetry (DSC) techniques. Two types of samples were prepared from bisphenol-A type epoxide monomer: one had a crosslinked structure cured by 4,4′-diaminodiphenylmethane and the other had a linear uncrosslinked structure polymerized by aniline. Both specimens were compressed first in the rubbery state and subsequently compressed further in the glassy state. After compressed in the glassy state, the specimens were subjected to the TSSR and the DSC measurements. The TSSR results indicated that the amount of plastic strain recovering at temperatures below the glass transition temperature T_g was larger for the crosslinked sample than for the linear sample. On the other hand, the DSC results indicated that the amount of exothermic heat flow at temperatures below T_g was less for the crosslinked sample than for the linear sample. These results presumably indicate that, for the crosslinked epoxy glass deformed in the glassy state, glass-like strain recovers quite cooperatively with rubber-like strain at temperatures below T_g, in contrast to independent recovery of these strains in the linear epoxy glass.     

Synthesis and characterization of graft copolymers of syndiotactic polystyrene with polybutadiene and 4‐methylstyrene

The basic method for synthesizing syndiotactic polystyrene‐g‐polybutadiene graft copolymers was investigated. First, the syndiotactic polystyrene copolymer, poly(styrene‐co‐4‐methylstyrene), was prepared by the copolymerization of styrene and 4‐methylstyrene monomer with a trichloro(pentamethyl cyclopentadienyl) titanium(IV)/modified methylaluminoxane system as a metallocene catalyst at 50°C. Then, the polymerization proceeded in an argon atmosphere at the ambient pressure, and after purification by extraction, the copolymer structure was confirmed with ¹H‐NMR. Lastly, the copolymer was grafted with polybutadiene (a ready‐made commercialized unsaturated elastomer) by anionic grafting reactions with a metallation reagent. In this step, poly(styrene‐co‐4‐methylstyrene) was deprotonated at the methyl group of 4‐methylstyrene by butyl lithium and further reacted with polybutadiene to graft polybutadiene onto the deprotonated methyl of the poly(styrene‐co‐4‐methylstyrene) backbone. After purification of the graft copolymer by Soxhlet extraction, the grafting reaction copolymer structure was confirmed with ¹H‐NMR. These graft copolymers showed high melting temperatures (240–250°C) and were different from normal anionic styrene–butadiene copolymers because of the presence of crystalline syndiotactic polystyrene segments. Usually, highly syndiotactic polystyrene has a glass‐transition temperature of 100°C and behaves like a glassy polymer (possessing brittle mechanical properties) at room temperature. Thus, the graft copolymer can be used as a compatibilizer in syndiotactic polystyrene blends to modify the mechanical properties to compensate for the glassy properties of pure syndiotactic polystyrene at room temperature. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

The effect of molecular weight and casting solvent on the miscibility of polystyrene-poly(α-methyl styrene) blends

The glass transition temperatures (T_g) have been measured for blends of polystyrene and poly(α-methyl styrene) in the molecular weight ranges: polystyrene, 2030 < M < 250 000, and poly(α-methyl styrene), 17 000 < M < 250 000. The presence of either one T_g or two has been used as a criterion to determine the miscibility of each blend over a range of compositions, and the T_gs were obtained from measurements of the temperature dependence of the heat capacities of the blends. A sinlge T_g was observed over the entire composition range for blends in which the component molecular weights were both less than 70 000 g mol^?1, when these were cast from toluene solutions. When propylene oxide solutions were used to prepare the blends, this limit dropped to M = 50 000 g mol^?1. By using the appearance of two T_gs as an indication that phase separation had taken place, it was possible to establish regions of miscibility and immiscibility as a function of casting solvent and molecular weight for both components. The change in heat capacity at the glass transition was measured and it was found that for miscible blends the heat capacity changes are similar to the calculated values, but for immiscible systems the measured change is smaller than expected.     

Lindemann-like size-independent glass-transition criterion for polymers

It is well known that the intrinsic melting mechanism is independent of crystal size according to Lindemann's melting criterion. In order to probe whether the glass transition mechanism is also size-independent, segment dynamics of free-standing polystyrene (PS) films is determined by considering the temperature- and thickness-dependent number of styrene segments N_α(T,D) in the cooperative rearranging region (CRR). Under the help of Adams-Gibbs glass transition theory and molecular dynamics simulation, N_α(T,D) function is established and it decreases as D decreases or T increases. However, N_α[T_g(D),D] at the glass transition temperature T_g(D) is size-independent, which is consistent with the simulation results obtained by Donth's method. Meanwhile, its relative temperature function N_α{[T − T_g(D)]/T_g(D)} is also size-independent. Therefore, N_α[T_g(D),D] function as a criterion for glass transition, which describes the physical nature of the glass transition, is similar to the vibrational amplitude in Lindemann's melting criterion.     

Photo-Crosslinked Poly(ε-caprolactone fumarate) Networks: Roles of Crystallinity and Crosslinking Density in Determining Mechanical Properties

We present a material design strategy of combining crystallinity and crosslinking to control the mechanical properties of polymeric biomaterials. Three polycaprolactone fumarates (PCLF530, PCLF1250, and PCLF2000) synthesized from the precursor polycaprolactone (PCL) diols with nominal molecular weights of 530, 1250, and 2000 g mol⁻¹, respectively, were employed to fabricate polymer networks via photo-crosslinking process. Five different amounts of photo-crosslinking initiator were applied during fabrication in order to understand the role of photoinitiator in modulating the crosslinking characteristics and physical properties of PCLF networks. Thermal properties such as glass transition temperature (T_g), melting temperature (T_m), and degradation temperature (T_d) of photo-crosslinked PCLFs were examined and correlated with their rheological and mechanical properties.