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Physical properties of polymers
[edit]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
[edit]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
[edit]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.
A synthetic polymer may be described as crystalline if it contains regions of three-dimensional ordering on atomic (rather than macromolecular) length scales, usually arising from intramolecular folding and/or stacking of adjacent chains. Synthetic polymers may consist of both crystalline and amorphous regions; the degree of crystallinity may be expressed in terms of a weight fraction or volume fraction of crystalline material. Few synthetic polymers are entirely crystalline.[1]
Phase transitions
[edit]The term "melting point" when applied to polymers suggests not a solid-liquid phase transition but a transition from a crystalline or semi-crystalline phase to a solid amorphous phase. Though abbreviated as simply "Tm", the property in question is more properly called the "crystalline melting temperature". Among synthetic polymers, crystalline melting is only discussed with regards to thermoplastics, as thermosetting polymers will decompose at high temperatures rather than melt.
The boiling point of a polymer substance is never defined, in that polymers will decompose before reaching assumed boiling temperatures.
A parameter of particular interest in synthetic polymer manufacturing is the glass transition temperature (Tg), which describes the temperature at which amorphous polymers undergo a second order phase transition from a rubbery, viscous amorphous solid to a brittle, glassy amorphous solid. The glass transition temperature may be engineered by altering the degree of branching or cross-linking in the polymer or by the addition of plasticizer.[2]
Polymer structure
[edit]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
[edit]Cross-linking
[edit]Anomalous linkage
[edit]===Tacticity
Properties of polymers
[edit]Expressions of mass or size
[edit]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
[edit]
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
[edit]Lamp-based RTP
[edit]Lamp-based RTP (second/millisecond)
Hotwall design=
[edit]Esp. Axcelis where wafer takes an elevator to the process chamber.
Laser RTP
[edit]Millisecond - full melt?
Temperature Control
[edit]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
[edit]Dopant activation
Thermal Oxidation
Metal reflow
Manufacturers of RTP Equipment
[edit]- Advanced Energy Thermal Instrumentation
- Applied Materials
- Axcelis Technologies
- Dainippon Screen Electronics (DNSE)
- Koyo Thermal Systems
- Mattson Technology
- Modular Process Technology
- Ultratech
Additional information
[edit]Semiconductor International article on RTP technology
See Also
[edit]
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Physical properties of polymer molecules
[edit]Expressions of mass or size
[edit]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
[edit]Optical activity or stereochemistry
[edit]- 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
[edit]polyethylene polypropylene polyvinyl chloride polyethylene terephthalate polystyrene polycarbonate
Polymers in nature
[edit]polypeptides/proteins polynucleic acids (DNA/RNA) rubber
Polymer science
[edit]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.
- ^ http://www.iupac.org/publications/books/pbook/PurpleBook-C4.pdf
- ^ Brandrup, J.; Immergut, E.H.; Grulke, E.A.; eds Polymer Handbook 4th Ed. New York: Wiley-Interscience, 1999.
- ^ Flory, P.J. and Vrij, A. J. Am. Chem. Soc.; 1963; 85(22) pp3548-3553