4) I saperi tradizionali della gondola veneziana
2.1 La dimensione immateriale del patrimonio culturale nelle scienze sociali del XX secolo: un apporto multidisciplinare
Objective:
After reading this unit, you should able to Know what polymer is?Know what chemical and physical properties of polymer>
Know the polymer science keywords.
Reading 1.
Nowadays, polymers play an important part in human living. Polymer commodity are used everywhere from household utilities to industrial products. The first polymers were natural products that date back to antiquity, especially cotton, starch, proteins and wool. Beginning early in the twentieth century, synthetic polymer was made, showed a tremendous possibility of the new material. Hence, polymer chemistry was born to meet the requirement of modern society making and understanding new kinds of plastics, rubber, adhesive, fiber, coatings and so on.
Polymers are considered as ―macromolecules‖. Chemically, polymers are used to describe long-chain molecules of very high molecule weight. There are many type of polymer including natural polymer and synthetic polymer. In there, a linear polymer consists of a long chain of skeletal atoms to which are attached the substituent groups. Polyethylene is one of the simplest examples. Linear polymer exists as elastomer, flexible materials, or glasslike thermoplastic. Another kind of polymer is branched polymer, can be visualized as a linear polymer with branches of the same basic structure as the main chain. They can be distinguished linear polymer by their lower tendency to crystallize or by their different solution viscosity or light scattering behavior. Linear polymers and branched polymers are usually soluble in some solvent. A crosslinked or network polymer is one in which chemical linkages exist between the chains.
Such materials are usually swelled by solvents, but they do not dissolve. In fact, this insolubility can be used as a cautious criterion of a crosslinked structure. Actually, the amount by which the polymer is swelled by a liquid depends on the density of crosslinking: the more crosslinks present, the smaller is the amount of swelling. If the degree of crosslinking is high enough, the material is maybe a rigid, high-melting, unswellable solid.
115 TASK 1. COMPREHENSION QUESTIONS
Answer the questions below.1. What is macromolecules?
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2. How many types can polymers can be rudely classified?
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3. Can you give some characteristics of linear polymer?
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4.
Is the solution viscosity of linear polymer is higher than branched polymer?...
5. Which property of polymer is used to show cross-linked structure.?
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6. when does the polymer become rigid material?
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Reading 2.
At bulk state, sometimes called the condensed or solid state, includes both amorphous and crystalline polymers. Amorphous polymers are materials with no ordered molecular chains. These materials are typically transparent and exhibit a board range of melting temperature. While the later are materials with an ordered structure. They exhibit a discreet melting point and are typically non-transparent with good to excellent chemical resistance.
The melting of polymers may be observed by any of several experiments. Most long-chain polymer show a characteristic sequence of changes as they are heated. All linear polymer are glasses at low temperature.
As the temperature is raised, a certain point is reached at which the polymer changes from a glass to a rubber. This change is known as the glass transition temperature (Tg). When heated above Tg, amorphous polymer pass successively through rubbery, gumlike, and finally liquid states with no clear demarcation between different phase. On the other hand, crystalline polymer remain flexible and thermoplastic above Tg until the temperature is raised to the crystalline melting temperature, Tm. At this point the polymer melts to a viscous liquid and a sharply define temperature. On melting, the sample becomes clear, or more transparent. For example, amorphous materials are similar to a stick of butter. When heated, a solid stick of butter begins to soften. As additional heat is applied, more of the stick softens and some areas begin to melt. Finally, as enough heat is applied the stick melts completely. Crystalline and semi-crystalline materials, on the other hand, are similar to an ice cube. As heat is applied to the ice cube, it begins to melt.
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The ice cube can only exist as a solid ice cube or as liquid water. The soft state does not exist. In both cases, the stick of butter and the ice cube started as solids and ended as liquids, but the stick of butter experienced a board temperature range in which it softened prior to melting completely.
Elementary physics divides matter into three quite distinct categories: solids, liquids, and gases. Solids are substances that occupy a fixed shape and volume (i.e., they do not flow). Liquids flow readily flow readily by occupy a fixed volume. Gases flow and change their volume easily. This simple view of the universe does not account for the properties of open-chain or moderately crosslinked polymer. Most polymers are neither classical solids nor liquids. They are viscoelastic materials. The viscoelastic state has the characteristics of both the solid and liquid states. Consider a piece of lightly crosslinked natural rubber. At rest on a laboratory bench, it has all the characteristics of a solid (definitely shape, fixed volume, no evidence or liquid flow). But if we stretch the material or apply pressure to one part of it, it will change shape like a liquid. Of course, if we release the tension or pressure, it will revert to its original shape, sometime quickly, but, for some material, over a long period of time. These are some of the characteristics of the viscoelastic state, and these unusual properties account for the valuable properties of macromolecules. By possessing these properties, polymers can be used for many applications where conventional solids (like sodium chloride or benzoic acid) or ordinary liquids would be unsuitable.
TASK 2: TRUE OR FALSE
Decide whether the following statements are true (T) or false (F).
1. Amorphous and crystalline are two types of condensed state.
2. Amorphous polymer is a transparent material.
3. Amorphous polymers are materials with an ordered structure.
4. The ―later‖ in line four refers to amorphous.
5. All branched polymer are glasses at low temperature.
6. Both amorphous and crystalline polymers are flexible and thermoplastic when they are heated at the temperature above Tg.
7. Crystalline and semi-crystalline materials do not exit as a solid and liquid mixture.
8. Polymers are mostly considered as viscoelastic materials.
9. Open-chain or moderately crosslinked polymers are considered as viscoelastic materials
10. Viscoelastic materials can not restore their shape when tension or pressure acting on them releases.
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Reading 3.Rheology is the study of the deformation and flow of the matter. Polymer latexes and suspensions are aqueous dispersions with viscosities dependent on solid content and additives. Polymer solutions may be much more viscous, depending on the concentration, molecular weight, and temperature. The rate of heat generated during viscous flow may be significant. Thus the heat generated may actually reduce the viscosity of the material. In the vast majority of case, the viscosity decreases with increasing shear rate, giving rise to what is often called ―shear-thinning‖ which is typical for pseudoplatic materials. On the other hand, dilatants materials exhibit ―shear thicken‖, which is opposite with pseudoplastic material. It must be remember that all macroscopic deformations of matter depend ultimately on molecular motion. In the case of high polymers, the chain‘s radius of gyration is changed during initial deformation or flow.
Thermal motions tend to return the polymer to its conformation, thus raising its entropy. Clearly, there is a direct relationship between the mechanical or viscous behavior of polymeric material and their molecular behavior.
A major test of the mechanical behavior of polymer, especially those plastic below their glass transition temperature, involves the measurement of tensile strength. While it can be argued that tensile strength is not the best quantity to characterize engineering behavior, it is simple, inexpensive, and very widely reported. Often crystalline plastics have higher elongations to break than amorphous material because the crystalline regions act as reinforcement. The thermosetting materials are almost always amorphous.
Tensile strength and elongation at break, usually increase with the molecular weight of the polymer, decrease with branching, increase with increasing crystallinity, and for the thermosetting materials, decrease with very high levels of crosslinking.
TASK 3. COMPREHENSION QUESTIONS
Answer the questions below.1. What factors will affect the viscosities of polymer latex?
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2. How can the viscosity of the material be influenced by the temperature?
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3. Why we should measure the tensile strength of polymeric materials?
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4. Which factors do affectTensile strength of thermosetting material?
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5. Whether the tensile elongations to break of crystalline plastics higher than that of amorphous material ? ...
6. What is dilatants materials?
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TASK 4: MATCHING
Match a word or phrase in A with its definition in B
A B
1) thermoplastic a) the reversible transition in amorphous materials (or in amorphous regions within semicrystalline materials) from a hard and relatively brittle state into a molten or rubber-like state.
2) thermoset b) a transition from a crystalline or semi-crystalline phase to a solid amorphous phase
3) glass-transition temperature c) polymer material that irreversibly cures. The cure may be done through heat
4) Melting point of a polymer d) are additives that increase the plasticity or fluidity of the material to which they are added; these include plastics, cement, concrete, wallboard, and clay. Although the same compounds are often used for both plastics and concretes the desired effects and results are differen
5) co-polymer e) a polymer that turns to a liquid when heated and freezes to a very glassy state when cooled sufficiently
5) plasticilizer f) a polymer derived from two (or more) monomeric species, as opposed to a homopolymer where only one monomer is used.
TASK 5. Gap Filling
Complete the table below with appropriate words or phrases
polymers branched backbone linearity homogeneous
cross-linked degree matrix folding polydispersity
coexist bulk homopolymers copolymers celluse
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Understanding polymeric materialsPolymeric materials can be considered to be mixture of (1)……. with various chain lengths or molecular weights. While the term ―polymer‖ in popular usage suggests ―plastic,‖ polymers comprise a large class of natural and synthetic materials with a variety of properties and purposes. Natural polymer materials such as shella and amber have been in use for centuries. Biopolymers such as protein (for example, hair, skin, and bone components) play crucial roles in biological processes. Other natural polymer include (2)……., which is the main constituent of wood and paper.
Polymeric materials can be described by: (1) structural properties directly related to the physical arrangement of the monomers along the (3)……. of the polymer chain at the nano- or microscopi scale, (2) the morphology of the polymer (4)……. at the mesoscopic scale, and (3) the (5)……. behavior at the macroscopic scale.
The structural properties of a polymer can be characterized by monomer type(s), and the chain (6)…….
and size (or length). Polymers that contain a single monomer type are (7)……., while those with two or more type are (8)…….. For example, polystyrene is a homopolymer composed of styrene monomers, while ethylene-vinyl acetate is a copolymer containing ethylene and vinyl acetate monomers. Also, polymers are classified as linear or (9)……. with the former having a linear backbone structure. The latter have a backbone chain and one or more side chains that form sar, brush, or comb structures. The are often highly (10)……. polymers involving four or more distint polymeric chains, often referred to as polymer networks. At an extreme, a networks ecome more complex polymer gels are formed where all chains have links ino a single molecule. One fascinaing example is a aerogel, whose refrative index approaches that of aire, due to its largely hollow structure.
The degree of polymerization determines the chain lengths, that is, the number of monomer units that form the chain. Because polymer chains are not (11)……. in length their molecular weights ae expresses statisticlly using weight-or number – average, molecular – weight distributions. The ratio of these two molecular – weight measures is the so – called (12)……. index, which describes the breadthe of e distribution.
A synthesic polymer may contain both crystalline and ammorphous phases, which can (13)……. in regions of the polymer. The crystalline phase is composed of a three-dimensional ordering at the atomic
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length scales, as a result of intramolecular (14)……. or stacking of the adjacent chains. The (15)……. is crystallinity is expressed as weight or volume fration of the crystalline phase. Amorphous polymers contain no crystalline regions. Most synthetic polymers are either amorphous or semi-crystalline.
TASK 6: How to find good keywords
Find the keywords in reading paragraphs 2 and 3
TASK 7: Summary
In about 5 sentences, summarize the main idea in paragraphs 2 and 3
TASK 8: Glossary
Search your knowledge, look up your dictionary, internet or ask your instructor to clarify the definition and Vietnamese meaning of the following terminologies.
No Terminology Definition Vietnamese
1 adhesive 2 amorphous 3 coating 4 conformation 5 co-polymer 6 crosslinked 7 crystallinity 8 deformation 9 elastomer 10 elongation 11 glass transition
temperature 12 glasslike
thermoplastic 13 homopolymers 14 linear polymer 15 melting temperature 16 plasticilizer
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17 plastics18 polymer 19 Rheology 20 shear rate 21 soften 22 starch 23 tensile 24 thermoplastic 25 unswellable 26 viscoelastic