摘要:

AbstractStress‐induced crystallization in a rapidly stretched natural rubber gum vulcanizate has been studied using thermal techniques to follow the development of crystallinity. A special‐purpose analog computer has been assembled and used on‐line to process the thermal and mechanical data obtained in high speed tensile testing. Roughly first‐order room temperature crystallization kinetics curves were obtained having time constants of 50–60 msec in the range of 400–540% extension. While the rate of this rapid, presumably primary crystallization appears rather insensitive to elongation in this limited range, the extent of crystallization at 400 msec increases smoothly from zero at 340% elongation to around 18% at 540% elongation. It is shown that our high‐speed tensile tester can stretch this vulcanizate fast enough that most of the crystallization takes place after extension has been completed. Stress‐strain curves obtained at this high rate are compared with those obtained at lower rates where crystallization takes place during

ISSN:0449-2978    

DOI:10.1002/pol.1968.160061001

出版商:John Wiley&Sons, Inc.    

年代:1968

数据来源: WILEY

摘要:

AbstractWe studied the high‐temperature mechanical behavior of unplasticized poly(vinyl chloride) by stress relaxation experiments covering a temperature range of 100–180°C and a time interval of 0.01–3600 sec at an elongation of 40%. The polymer was observed to respond elastomerically within these set temperature limits. The molecular basis for this behavior is linked to the partial crystalline nature of this material. The crystalline phase is postulated to be the seat of the relaxation process taking place in the rubbery response region, and evidence is presented supporting this point of view. A comparison is made between free volume variations arising from thermal expansion and those originating instead from uniaxial straining. It is also pointed out that, following linear viscoelastic theory, the viscosity is a time‐dependent quantity, and it is shown that for this polymer its variation with temperature at short times is ninefold less than the corresponding variations in its limiting value. Finally it is noted that the measured high‐temperature tensile strength corresponds to a chain bond strength approximately 1/300 of the theoret

ISSN:0449-2978    

DOI:10.1002/pol.1968.160061002

出版商:John Wiley&Sons, Inc.    

年代:1968

数据来源: WILEY

摘要:

AbstractA simple theoretical development is proposed to explain the variation of birefringence with degree of crystallization for crosslinked, stretched polymer networks. The model adopted for the calculations is one proposed by Flory in order to calculate the thermodynamic properties of such systems and hence is restricted to the special case of network orientation prior to the onset of crystallization. Our findings are briefly discussed and possible applications of the results are considered.

ISSN:0449-2978    

DOI:10.1002/pol.1968.160061003

出版商:John Wiley&Sons, Inc.    

年代:1968

数据来源: WILEY

摘要:

AbstractThe orientation of the crystallographiccaxis (chain axis) was followed by x‐ray diffraction during the crystallization of four samples of isotactic polystyrene differing in elongation ratio. The crystallite orientation can be expressed by 〈cos2χc〉, where χcis the angle between thecaxis and the stretching direction. The degrees of crystallinitywwere estimated from the diffraction data by using density for calibration. It was found that 〈cos2χc〉 decreases in a linear manner with crystallinity, the rate of decrease being very small when the elongation ratio α is 5, but becoming progressively larger as α is decreased toward unity. A qualitative measure suggests that amorphous orientation decreases during crystallization at a rate which is nearly independent of α. The variation of 〈cos2χc〉 withwis therefore governed by the orientation of the statistical chain segments prior to crystallization. If the elongation ratio is small, the supply of well oriented statistical segments is limited, and 〈cos2χc〉 will decrease at a rapid rate during crystallization. A treatment due to Krigbaum and Roe permits evaluation of the ratio, ν/N, where ν andNare the average numbers of statistical segments in the crystallization nucleus of critical size, and in a network chain, respectively. Our polystyrene samples were not crosslinked, so chain entanglements must serve as junction points. Values of ν could not be obtained, sinceNwas unknown. However, the (ν/N) ratio for isotactic polystyrene decreases slowly with α, and the values agree reasonably well with those obtained in a previous study of oriented polychloroprene networks. After nearly complete crystallization (ω ca. 0.30), the long period spacing measured by low angle diffraction was approximately 135 Å, and varied only slightly with elongation ratio in the range α = 1 to 5. It therefore appears that chain folded lamellae are present in both drawn and undrawn

ISSN:0449-2978    

DOI:10.1002/pol.1968.160061004

出版商:John Wiley&Sons, Inc.    

年代:1968

数据来源: WILEY

摘要:

AbstractThe Rayleigh‐Gans theory of light scattering by a homogeneous cylinder has been applied to the case of scattering by a pad of fibers immersed in a liquid. The theory enables one to locate precisely the matching wavelength at which the mean refractive index of a sample of optically heterogeneous fibers coincides with that of an immersion medium. Thereby it offers a method of determining the mean refractive index. Besides, it facilitates a quantitative estimation of the variability of refractive index. The new technique has been used to study the birefringence of untreated cotton and ramie. The results are presented and discusse

ISSN:0449-2978    

DOI:10.1002/pol.1968.160061005

出版商:John Wiley&Sons, Inc.    

年代:1968

数据来源: WILEY

摘要:

AbstractExperimental evidence concerning the dependence of the intrinsic viscosity [η] on molecular weightMin the low molecular weight range (from oligomers toM= 5 × 104) has been collected in a variety of solvents for about ten polymers, i.e., polyethylene, poly(ethylene oxide), poly(propylene oxide), polydimethylsiloxane, polyisobutylene, poly(vinylacetate), poly(methyl methacrylate), polystyrene, poly‐α‐methylstyrene, and some cellulose derivatives. In theta solvents, the constancy of the ratio [η]Θ/M0.5extends down to values ofMmuch lower than those predicted by current hydrodynamic theories. In good solvents, and on decreasingM, the polymers examined, with the exception of polyethylene and some cellulose derivatives, show a decrease in the exponentaof the Mark‐Houwink equation [η] =KMa. This upward curvature gives rise to the existence of a more or less extended linear region where the equation [η] =K0M0.5is obeyed. Below the linear range, i.e., for even shorter chains, the exponentacan increase, i.e., polydimethylsiloxane, or decrease below 0.5, i.e., poly(ethylene oxide), depending on the particular chain properties. These different dependences have been discussed in terms of: (a) variations of thermodynamic interactions with molecular weight; (b) variations of conformational characteristics (as for instance the ratio) 〈r02/nl2〉, where 〈r02〉 is the unperturbed mean square end‐to‐end distance andnis the number of bonds each of lengthl; (c) hydrodynamic pr

ISSN:0449-2978    

DOI:10.1002/pol.1968.160061006

出版商:John Wiley&Sons, Inc.    

年代:1968

数据来源: WILEY

摘要:

AbstractEither of the two endothermic melting peaks found by differential thermal analysis of nylon 66 may be converted to the other by appropriate choice of annealing conditions. The two peaks are considered due to the melting of two morphological species, forms I and II. Form I is relatively fixed in melting temperature, while the form II melting temperature varies with annealing conditions and can be either above or below form I. The two forms can be distinguished by whether or not the conversion I → II takes place; if the sample is in form II no change in the thermogram is observed under suitable conversion conditions. The conversion of form I to form II also takes place during cold drawing. It has been previously shown that form I results from rapid cooling from the melt, and form II results from slow cooling. Form I appears to be kinetically favored, while form II is thermodynamically preferred. The variability in the form II melting point is attributed to variable crystal size and/or perfectio

ISSN:0449-2978    

DOI:10.1002/pol.1968.160061007

出版商:John Wiley&Sons, Inc.    

年代:1968

数据来源: WILEY

摘要:

AbstractUsing the criterion that a coupling entanglement in a polymer network is trapped if all four strands radiating from it terminate in chemical cross‐links (an approximation to a more rigorous treatment of Langley), an equation is derived relating the equilibrium modulus to the magnitude of the compliance in the frequency region where all coupling entanglements, whether trapped or not, contribute to the elasticity. The latter value is estimated from the storage compliance at the frequency where the storage compliance of the uncross‐linked polymer corresponds to the entanglement compliance derived from integration over the loss compliance. The theory agrees rather well with data on vulcanizates of natural rubber. For 1,4‐polybutadiene and styrene–butadiene rubbers, the agreement is somewhat less satisfactory, but the results support the hypothesis that the low‐frequency losses observed in lightly cross‐linked rubbers are due to relaxation of untrapped en

ISSN:0449-2978    

DOI:10.1002/pol.1968.160061008

出版商:John Wiley&Sons, Inc.    

年代:1968

数据来源: WILEY

摘要:

AbstractThe spherulitic growth rate of isotactic polystyrene has been measured in a wide range of temperature by means of a polarizing microscope provided with a hot stage. It was possible to fit the experimental data to theory by choosing a value of 75 for the constantC2of the WLF equation. The growth rate parameters were compared with those of polyethylene and polychlorotrifluoroethylene. The slowness of crystallization of isotactic polystyrene is mainly a consequence of the lower mobility of the molecules caused by the bulky phenyl groups.

ISSN:0449-2978    

DOI:10.1002/pol.1968.160061009

出版商:John Wiley&Sons, Inc.    

年代:1968

数据来源: WILEY

ISSN:0449-2978    

DOI:10.1002/pol.1968.160061010

出版商:John Wiley&Sons, Inc.    

年代:1968

数据来源: WILEY