ISSN:0360-8905    

DOI:10.1002/polc.5070590103

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1977

数据来源: WILEY

摘要:

AbstractAn outline is given of heterogeneous nucleation as the main starting point for crystallization of unoriented polymer melts and solutions. The influence of temperature history on nucleation is discussed. As little is known about naturally occurring nucleants, the nature of several types of artificial nucleating agents is described. Potential mechanisms of heterogeneous nucleation are discussed against the background of various theoretical models. Experimental methods of measurement of nucleation density or of nucleation frequency may give some indication as to which mechanism is operative. The main conclusion is that heterogeneous nucleation is governed by “active sites” in the surface of the nuclea

ISSN:0360-8905    

DOI:10.1002/polc.5070590104

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1977

数据来源: WILEY

摘要:

AbstractCharacteristic features of isothermal growth, thickening, and melting of melt‐grown single crystals of poly(ethylene oxide) fractions, as reported in a recent paper, are complemented and further discussed. A newly discovered phenomenon–isothermal superheating–is described and its implications are discussed. Growth and thickening rate data are analyzed in terms of the two independent molecular parameters which determine the lamellar thickness: the number,n, of folds per molecule and the total chain length, λ Current formulations of kinetic theories are critically analyzed in view of their applicability to the present growth rate data. The analysis reveals that, for the low‐molecular‐weight fractions, the theory only applies to fast growth obtained at high undercooling. At low undercooling, where molecules deposit in extended orn‐times folded‐chain conformation, however, growth proceeds by a nucleation mechanism which appears to be quite different from that implied

ISSN:0360-8905    

DOI:10.1002/polc.5070590105

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1977

数据来源: WILEY

摘要:

AbstractThis paper gives a brief account of the areas in which fibrillar crystals have been encountered and provides a mechanism for the formation of bundle‐like nuclei and the longitudinal growth of fibrillar polymer crystals from flowing solution

ISSN:0360-8905    

DOI:10.1002/polc.5070590106

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1977

数据来源: WILEY

摘要:

AbstractThis paper discusses the morphology of individual lamellar crystals of semicrystalline linear high polymers in the bulk or oriented states. We discuss the simple two‐phase model, the model where the folds are level, and intermediate models. We also discuss the dependence of lamellas upon each other caused by the chains and fibrils which connect the crystals and the coalescence of the disordered parts of the lamella

ISSN:0360-8905    

DOI:10.1002/polc.5070590107

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1977

数据来源: WILEY

摘要:

AbstractThis paper discusses four general problem areas and their associated theories: the relationship between crystal growth and nucleation; the concentration dependence of crystal growth rates; the crystallization of random copolymers, and the annealing (thickening) behavior of polymer crystals.

ISSN:0360-8905    

DOI:10.1002/polc.5070590108

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1977

数据来源: WILEY

摘要:

AbstractOwing to their simple chemical structure paraffins are generally regarded as representing a system which is especially suitable for a model study of the thermal behavior of oligomeric and polymeric solids. Extensive investigations on the properties ofn‐tritriacontane (C33H68) provide an example. For this system an identification of several types of motional mechanisms become possible. Their presence leads to peculiarities in the thermal expansion and to the occurrence of solid‐solid phase transitions.Paraffins and polyethylene exhibit a marked anisotropy in the lateral thermal expansion. An analysis of the molecular background can be based on a modified Grüneisen theory which connects expansion coefficients with elastic constants and Grüneisen constants. The Grüneisen constants describe anharmonic properties of the lattice vibrations and can be derived from a temperature dependent measurement of the frequencies of a number of Raman active lattice modes. The analysis shows that the observed anisotropy is mainly a consequence of the uniaxial orientation of the strong thermal pressure originating from the rotational and torsional oscillations of the chains.In the case of C33H68, the transition from the well ordered crystalline state to the melt occurs in several consecutive steps. Three solid‐solid phase transitions occur before melting and each one leads to a steplike decrease in the degree of order. Applying a combination of x‐ray and spectroscopic techniques it was possible to follow on a molecular level the changes in the state of order by specifying the defect structure. It is found that coupled 180°‐jumps, flip‐flop screw jumps, and defect diffusion along a chain set in successively, the onset of each mechanism of motion being accompanied by a phase transition.The measured melting entropy is indicative for a coiled chain conformation in the melt with slight deviations from a random coil owing to the occurrence of limited short range order. This follows from theoretical

ISSN:0360-8905    

DOI:10.1002/polc.5070590109

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1977

数据来源: WILEY

ISSN:0360-8905    

DOI:10.1002/polc.5070590102

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1977

数据来源: WILEY