The rheological behavior of blends of poly(styrene-co-acrylonitrile) (SAN) and poly(ε-caprolactone) (PCL) was investigated, using a cone-and-plate rheometer. For the study, blends of various compositions were prepared by melt blending using a twin-screw compounding machine. The rheological properties measured were shear stress (σ12), viscosity (η), and first normal stress difference (N1) as functions of shear rate (γ) in steady shearing flow, and dynamic storage modulus (G′) and loss modulus (G″) as functions of angular frequency (ω) in oscillatory shearing flow, at various temperatures. It has been found that logarithmic plots of N1 versus σ12, and logarithmic plots of G′ versus G″, become virtually independent of temperature but vary regularly with blend composition, and that the zero-shear viscosity of the blends, (ηo)blend, follows the relationship, 1/log(ηo)blend = wA/log η0A + wB/log η0B, where η0A and η0B are the zero-shear viscosities of components A and B, respectively, and wA and wB are the weight fractions of components A and B, respectively. The physical implications of the relationship found are discussed. 相似文献
Stereo diblock polylactides (SDB‐PLAs) composed of relatively short poly(d ‐lactide) (PDLA) segments and relatively long poly(l ‐lactide) (PLLA) segments were synthesized to have a wide number‐average molecular weight (Mn) range of 2.5 × 104–2.0 × 105 g mol?1 and d ‐lactyl unit content of 0.9–38.6%. The effects of incorporated short PDLA segments (Mn = 2.0 × 103–7.7 × 103 g mol?1) on crystallization behavior of the SDB‐PLAs were first investigated during heating after complete melting and quenching or during slow cooling after complete melting. Stereocomplex (SC) crystallites can be formed at d ‐lactyl unit content as low as 4.3 and 5.8% for heating and slow cooling, respectively, and for Mn of PDLA segments as low as 2.0 × 103 and 3.5 × 103 g mol?1, respectively. With decreasing Mn and increasing d ‐lactyl unit content, the cold crystallization temperature during heating decreased and the crystallization temperature during slow cooling increased. With increasing d ‐lactyl unit content, the melting enthalpy (ΔHm) of SC crystallites during heating and the crystallinity (Xc) of SC crystallites after slow cooling increased, whereas ΔHm of PLLA homo‐crystallites during heating and Xc of PLLA homo‐crystallites after slow cooling decreased. The total ΔHm of SC crystallites and PLLA homo‐crystallites during heating and the total Xc after slow cooling became a minimum at d ‐lactyl unit content of 10–15% and gave a maximum at d ‐lactyl unit content of 0%. Despite the accelerated crystallization of some of SDB‐PLAs, the low values of total ΔHm and Xc at d ‐lactyl unit content of 10–15% are attributable to the formation of two crystalline species of SC crystallites and PLLA homo‐crystallites. 相似文献
Softness parameters σM for cations and σX for anions, have been calculated as dimensionless quantities for approx. 90 cations and 18 anions. They are given by σM = [σA (Mm+) - σA(H+)]/σA(H+) and σX = [σB(Xa?) - σB(OH?)]/σA(H+) where σA = [σIi(M) + ΔH0h(Mm+)]/m and σB = [-Ea(X) + ΔH0h(Xa?)]/a are Ahrland's parameters. The new normalized and comparative (to the test ions H+ and OH?) softness parameters are positive for soft ions and negative for hard ones. These parameters, obtained independently, are used with a four-coefficient equation to calculate coordinate bond energies for metal halides with acceptable accuracy. Considerations of the average coordination in reciprocal molten salt mixture lead to an expression for the metathesis energy change as proportional to the product of the differences in softness parameters of the two cations and the two anions. An empirical one-coefficient equation involving the softness parameters is proposed to deal with next-nearest-neighbor interactions in binary common-ion molten salt mixtures. These relationships are then used with Blander and Topol's equation to predict the occurrence of irascibility gaps in uni-univalent reciprocal salt mixtures. The gaps found in other systems are also discussed in terms of the softness of the constituent ions. 相似文献
Conformational characteristics of poly(lactide)s have been investigated by density functional theory and ab initio molecular orbital (MO) calculations and NMR experiments on model compounds. Characteristic ratios, configurational entropies, and internal energies of poly(L-lactide) and poly(DL-lactide), whose stereosequences were generated by Bernoulli and Markov stochastic processes, were calculated under the refined rotational isomeric state scheme with conformational energies and geometrical parameters derived from the MO calculations. In terms of the conformational characteristics thus revealed, we have elucidated the reason why unperturbed chain dimensions determined experimentally for poly(L-lactide) are scattered considerably and, furthermore, discussed crystallization and crystal structures of poly(L-lactide) and molecular characteristics of poly(DL-lactide) synthesized from rac-lactide with stereospecific polymerization catalysts. 相似文献
The molecular and crystal structure of naturally occurring optically active poly(β-hydroxy-butyrate) was analysed by X-ray diffraction. The unit cell is orthorhombic, P212121-D42, with , , and and two molecules pass through the unit cell. The molecule has a conformation shown by the Fischer projection for the case of a rectus polymer, giving a left-handed () helix.The crystalline synthesized racemic polymer gave the same X-ray diffraction pattern as the naturally occurring optically active polymer. This result indicates that the racemic polymer has an isotactic configuration and consists of two kinds of crystallites, each composed only of the left-handed helices of rectus polymer chains, or only of the right-handed helices of sinister polymer chains. 相似文献
The effects of l-lactide dimer as additives on the crystallization behavior of poly(l-lactide) (PLLA) films were studied. Hence, neat PLLA films and PLLA containing l-lactide (5 % w/w) (PLLA/La) were prepared in dichloromethane at room temperature via solution casting. The non-isothermal cold crystallization of PLLA films were studied using differential scanning calorimetry at various heating rates including 2.5, 5, 7.5, 10 and 15 °C/min. However, the XC% was increased for PLLA/La films in comparison with neat PLLA films. The crystallization kinetics was then analyzed by the Avrami, Jeziorny, Ozawa and Mo kinetic models. It is found that all the kinetic models were established to describe the experimental data fairly well except the Ozawa model. The values of t1/2, ZC and F(T) indicated that the crystallization rate increased with increase in heating rates for PLLA and PLLA/La films. However, l-lactide dimer incorporated in PLLA films accelerates the crystallization process of PLLA at the high heating rate. The nucleation constant (Kg) and the surface free energy (σe) based on Lauritzen–Hoffman theory indicated that these parameters for PLLA/La films is lower than neat PLLA. 相似文献
A series of biodegradable poly (glycerol-sebacate-l-lactide) (PGSLA) copolymers, with variable PLLA length, were synthesized and characterized. The copolymers comprised PGS
backbone chain with a nominal molecular weight of 2,800 g/mol. The length of each PLLA side chain covered the 800–14,000 range,
while the length of the PLLA was easily controlled by the feed molar ratio of the l-lactide to the PGS. The structure of the copolymer was studied by nuclear magnetic resonance spectroscopy and gel permeation
chromatography. Differential scanning calorimetric measurements and thermal gravimetric analysis had been performed to indicate
the glass transition temperature (Tg), melting point (Tm), and the degree of crystallinity (χc). It was also found that the onset decomposition temperature (Td) of the copolymers was lower than those of the linear polylactide (LPLLA). After solution casting and solvent evaporation,
porous structures were found in the copolymer films by scanning electron microscope (SEM). Water contact angle results showed
that the hydrophilicity of the copolymers was much higher than that of linear PLLA. In vivo, PGSLA copolymer demonstrated
a favorable tissue response profile compared to PGS/LPLLA blend. There was also significantly less inflammation and fibrosis
during degradation. PGSLA might therefore serve as an excellent candidate material for medical applications, given its minimal
in vivo tissue response. 相似文献
Poly(L-lactide) (PLLA) oligo-esters with α-hydroxyl-ω-alkyl (alkyl = −CH2−[CH2−CH2]m−CH3, where m = 1, 2, 4, 5, 6, 7, 8, 9, and 10) end groups were synthesized by ring-opening polymerization of L-lactide (L-LA)
catalyzed by tin(II) 2-ethylhexanoate Sn(Oct)2 in the presence of aliphatic alcohols as initiators (HO−CH2−[CH2−CH2]m−CH3, where m = 1, 2, 4, 5, 6, 7, 8, 9, and 10). High yields (~ 62 to 71%) and Mn(NMR) in the range of 2120–2450 Da (PLLA) were obtained. Effects of alkyl end groups on thermal properties of the oligo-esters
were analyzed by DSC, TGA and SAXS. Glass transition temperature (Tg) gradually decreases with increase in the percent of−CH2−[CH2−CH2]m−CH3 end group, as results alkyl end group provides most flexibility to PLLA. An important effect of alkyl end group on a double
cold crystallization (Tc1 and Tc2) was observed, and is directly related with the segregation phase between alkyl end group and PLLA. TGA analysis revealed
that PLLA oligo-esters are more thermally stable with docosyl (−C22H45) respect to the butyl (−C4H9) end group, probably is due to steric hindrance of the end group (docosyl respect to butyl) toward intermolecular and intramolecular
transesterification. SAXS analysis showed that alkyl end group as docosyl restricted the growth of lamellae thickness (D)
due to steric hindrance. Characterization of hydroxyl and alkyl end groups in the PLLA oligo-esters was determined by MALDI-TOF,
GPC, FT-IR and 1 H and 13 C NMR. 相似文献