摘要:

One of the important aims of modern polymer research is to achieve an understanding of molecular chain conformation and of the spatial arrangement of the polymer molecules in the solid state. In attempts to solve this problem and to interpret the physical properties of high polymers, it is necessary to remember that a polymer structure must be considered as dynamic rather than as static. Due to its constitution, the structural system of a bulk polymer is not stable but goes toward its thermodynamic equilibrium by the conformational motions of chain elements changing the arrangement of macromolecules. The rate and spatial reach of molecular motions depend to a great extent on the temperature and, in fact, define the number of thermal transitions observed in polymers. The two most important thermal transitions are the glass-rubber transition, characterized by the temperatureTg, and the melting of crystalline elements, characterized by the temperature T, (in semicrystalline polymers). BelowTgthe material is in its hard glassy state, and molecular motions are restricted to side chains or to slow, short-range conformational movements of the amorphous phase. At higher temperatures, aboveTg, which is the onset of segmental mobility of backbone chains in the amorphous regions, the material is in a soft rubbery state. Increased segmental mobility allows certain structural transformations; for example, increased chain packing and a crystallite growth. AboveTm, crystallites are molten and the whole material is in a liquid state of high viscosity.

ISSN:1532-1797    

DOI:10.1080/15321799208021428

出版商:Taylor & Francis Group    

年代:1992

数据来源: Taylor

摘要:

Poly(4-methyl-l-pentene) (PMP) is a linear α-polyolefin with isobutylbranches and with very interesting properties. From a practical point of view. PMP presents high optical transparency. relatively high thermal stability and chemical resistance. in addition to excellent dielectric characteristics and microwave transparency. Consequently. a wide range of applications is possible for this polymer. The chemical resistance. temperature stability. and transparency provide uses in medical equipment such as hypodermic syringes. blood collection and transfusion equipment. pacemaker parts. and respiration equipment; in chemical laboratory equipment such as cells for spectroscopic and optical analysis; and in wire and cable coating. Its microwave transparency grants applications in microwave oven cookware, food packaging, and service trays.

ISSN:1532-1797    

DOI:10.1080/15321799208021429

出版商:Taylor & Francis Group    

年代:1992

数据来源: Taylor

摘要:

Polycondensation or step-growth polymerization systems differ from addition polymerization systems in that polymer growth occurs not just from monomer addition but from polymer-polymer reactions. Also, the reactions occurring during polycondensation often cause the production of a small molecule byproduct called a condensate.

ISSN:1532-1797    

DOI:10.1080/15321799208021430

出版商:Taylor & Francis Group    

年代:1992

数据来源: Taylor

摘要:

A contact lens is an optical device that is placed over the cornea of the eye in such a manner that the lens remains on the eye's surface throughout blinking. The main purpose of wearing a contact lens is to correct vision deficiencies; in this application they are called cosmetic lenses. Contact lenses can also be used medically for the treatment of certain diseases. In such cases they are called therapeutic or bandage lenses [l]. Contact lenses range from hard to soft. Hard lenses contain mainly poly(methy1 methacrylate) (PMMA) and are impermeable to oxygen. Hard and semirigid lenses permeable to oxygen are made from copolymers of siloxanes and methacrylates. Flexible, oxygen permeable lenses are made of silicones. Soft contact lenses are prepared from polymers that absorb large quantities of water to become hydrogels. The aqueous phase of the hydrogel is oxygen permeable. Hard and soft hydrophobic lenses require a relatively thick tear film between their posterior surface and the cornea of the eye. Soft hydrogel lenses adhere closely to the cornea with a tear film of only capillary thickness between the lens and the corneal surface. With any kind of contact lens, the cornea's surface must be wet and oxygenated at all times to remain transparent and healthy. The cornea is an avascular tissue with an active aerobic metabolism. Oxygenation of the corneal surface is normally accomplished through the tear film that supplies the oxygen to the epithelium of the cornea. When the eyelids are closed, oxygen is supplied to the cornea surface by blood capillaries of the palpebral conjunctiva (the posterior part of the eyelids). When the cornea surface is deprived of oxygen, epithelium glycogen decreases and lactic acid production increases as a result of anaerobic glycolysis [2]. The cornea then swells and its surface becomes hazy. Thus, with any type of contact lens, disruption of the Oz supply to the cornea surface must be minimized, either by oxygen-rich tear exchange under the lens, by oxygen permeation through the lens, or by both.

ISSN:1532-1797    

DOI:10.1080/15321799208021431

出版商:Taylor & Francis Group    

年代:1992

数据来源: Taylor

摘要:

This is a scanned image of the original Editorial Board page(s) for this issue.

ISSN:1532-1797    

DOI:10.1080/15321799208021427

出版商:Taylor & Francis Group    

年代:1992

数据来源: Taylor