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Physical properties of polymers
The properties of polymers vary dramatically depending on the nature, number, and arrangement of the constituent subunits. The same terminology used to describe the properties of non-polymer substances or molecules may be applied to polymers. For example, a polymer molecule may be described as polar, non-polar, or amphiphilic just as any other molecule. There are several cases, however, where a particu
Expressions of mass or size
Like any molecule, a polymer molecule may be described in terms of molecular weight or mass. In homopolymers or block copolymers, however, the molecular mass may be expressed in terms of degree of polymerization, essentially the number of monomer units which comprise the polymer or block. For synthetic polymers, the molecular weight is expressed in terms of statistics, to account for variability in molecular weight caused by uncertainty in the polymerization processes. Examples of such statistics include number-averaged molecular weight and weight-averaged molecular weight. The ratio of these two values is the polydispersity index, commonly used to express the overall variability in the molecular weight.
The space occupied by a polymer molecule is generally expressed in terms of radius of gyration or excluded volume.
=Expressions of crystallinity
When applied to polymers, the term crystalline has a somewhat ambiguous usage. In some cases, the term crystalline finds identical usage to that used in conventional crystallography. For example, the structure of a crystalline protein or polynucleotide, such as a sample prepared for x-ray crystallography, may be defined in terms of a conventional unit cell comprised of one or more polymer molecules with cell dimensions of hundreds of angstroms or more.
For linear polymers, especially synthetic polymers, crystallinity refers to a degree of three-dimensional ordering on the scale of a few angstroms. Since the dimensions of the polymer molecule are on the order of hundres of angstroms, a unit cell will include only small portions of individual polymer molecules. Crystalline regions in polyethylene, for example, are often regions where the polymer chain folds back on itself. These crystalline regions will be interspersed with amorphous regions with no long-range order.
Phase changes
Polymer structure
Describing the molecular structure of a polymer molecule is a difficult task owing to the large number of atoms involved and the myriad ways in which those atoms may be arranged. In addition to the conventional descriptors such as bond angle, molecular mass, and stereochemistry, polymer scientists have developed terminology to completely describe polymer molecular structure. These descriptors include information about the structural units, the relative arrangement of structural units, the nature of polymer branching, and intramolecular interactions between neighboring segements of the chain. Polymers with distinct biological function, such as proteins, often have a discipline-specific rubric for defining molecular structure.
Branching
Cross-linking
Anomalous linkage
===Tacticity
Properties of polymers
Expressions of mass or size
Polymers may b
The physical properties of a polymer, such as the glass transition temperature or tensile strength, depend on the nature of the constitutent molecules. Because of their size and complexity, polymer molecules may be manipulated in numerous ways to create substances with unique or custom-tailored properties.
=Molecular Weight
Rapid thermal processing (or RTP) refers to a semiconductor manufacturing process in which a silicon or other substrate is heated to high temperatures (1200 C or greater) in a time scale of several seconds or less. These processes are used for a wide variety of applications in semiconductor manufacturing including dopant activation, thermal oxidation, metal reflow and chemical vapor deposition.
RTP Equipment
Lamp-based RTP
Lamp-based RTP (second/millisecond)
Hotwall design=
Esp. Axcelis where wafer takes an elevator to the process chamber.
Laser RTP
Millisecond - full melt?
Temperature Control
One of the key challenges in rapid thermal processing is accurate measurement and control of the wafer temperature. Monitoring the ambient with a thermocouple is not feasible, in that the high temperature ramp rates prevent the wafer from coming to thermal equilibrium with the process chamber. One temperature control strategy involves in situ pyrometry to effect real time control.
RTP Applications
Dopant activation
Thermal Oxidation
Metal reflow
Manufacturers of RTP Equipment
- Advanced Energy Thermal Instrumentation
- Applied Materials
- Axcelis Technologies
- Dainippon Screen Electronics (DNSE)
- Koyo Thermal Systems
- Mattson Technology
- Modular Process Technology
- Ultratech
Additional information
Semiconductor International article on RTP technology
See Also
Physical properties of polymer molecules
Expressions of mass or size
Like any molecule, the molecular mass of a polymer molecule may be determined by summing the atomic masses of the constitent atoms. For polymers comprised of identical molecules, such as proteins or other biopolymers, the molar mass is the preferred expression of mass. In many polymers, however, especially synthetic polymers, the molecules which comprise the sample may not all have the same molecular mass. Thus, the mass is expressed statistically, generally in terms of a weight-averaged or number-averaged molecular mass. The ratio of the weight-averaged and number-averaged molecular weights is called the polydispersity index, and provides a measure of the uniformity of the polymer samples.
For polymers in solution, the space occupied by the polymer moleculeis generally expressed in terms of the radius of gyration (Rg).
Phase
Optical activity or stereochemistry
- degree of polymerization - The number of structural units in a polymer chain.
- molar mass distribution - The relationship between a polymer fraction and the molar mass of that polymer fraction.
- crystallinity - as well as the thermal phase transitions:
- Tg, glass transition temperature
- Tm, melting point (for thermoplastics).
- Branching - Polymer chains extending from the main polymer backbone chain.
- Stereoregularity or tacticity - the isomeric arrangement of functional groups on the backbone of carbon chains.
Polymers in commercial use
polyethylene polypropylene polyvinyl chloride polyethylene terephthalate polystyrene polycarbonate
Polymers in nature
polypeptides/proteins polynucleic acids (DNA/RNA) rubber
Polymer science
Most polymer research may be categorized as polymer science, a sub-discipline of materials science which includes researchers in chemistry (especially organic chemistry), physics, and engineering. Polymer science may be roughly divided into two subdisciplines:
- Polymer chemistry or macromolecular chemistry, concerned with the chemical synthesis and chemical properties of polymers.
- Polymer physics, concerned with the bulk properties of polymer materials and engineering applications.
The field of polymer science is generally concerned with synthetic polymers, such as plastics, or chemical treatment and modification of natural polymers.
The study of biological polymers, their structure, function, and method of synthesis is generally the purview of biology, biochemistry, and biophysics. These disciplines share some of the terminology familiar to polymer science, especially when describing the synthesis of biopolymers such as DNA or polysaccharides. However, usage differences persist, such as the practice of using the term macromolecule to describe large non-polymer molecules and complexes of multiple molecular components, such as hemoglobin. Substances with distinct biological function are rarely described in the terminology of polymer science. For example, a protein is rarely referred to as a copolymer.
- ^ Flory, P.J. and Vrij, A. J. Am. Chem. Soc.; 1963; 85(22) pp3548-3553