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Questions and Answers
What generally affects both the melting temperature Tm and the glass transition temperature Tg of a polymer?
What generally affects both the melting temperature Tm and the glass transition temperature Tg of a polymer?
How does the presence of chain ends and branches in a polymer affect its melting temperature Tm?
How does the presence of chain ends and branches in a polymer affect its melting temperature Tm?
In a high-molar-mass sample of polyethylene, the presence of branches can reduce the melting temperature Tm by how much?
In a high-molar-mass sample of polyethylene, the presence of branches can reduce the melting temperature Tm by how much?
What is a consequence of changing the structure of a polymer?
What is a consequence of changing the structure of a polymer?
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The relationship between the chemical potentials in the crystalline state and pure liquid state is expressed by which equation?
The relationship between the chemical potentials in the crystalline state and pure liquid state is expressed by which equation?
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What is the predicted slope of a plot of Tm against 1/l?
What is the predicted slope of a plot of Tm against 1/l?
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What distinguishes the melting behavior of semicrystalline polymers from other materials?
What distinguishes the melting behavior of semicrystalline polymers from other materials?
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How does annealing affect the melting temperature of crystalline polymers?
How does annealing affect the melting temperature of crystalline polymers?
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Which factor generally lowers the melting temperature Tm in polymers?
Which factor generally lowers the melting temperature Tm in polymers?
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How is the melting temperature of a polymer crystal affected by its crystallization temperature?
How is the melting temperature of a polymer crystal affected by its crystallization temperature?
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What does the intercept of the extrapolated lines on a Tm versus Tc plot represent?
What does the intercept of the extrapolated lines on a Tm versus Tc plot represent?
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What effect does a p-phenylene group have on Tm?
What effect does a p-phenylene group have on Tm?
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How does the presence of amide linkages affect the melting temperature Tm?
How does the presence of amide linkages affect the melting temperature Tm?
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What factor influences the observed melting temperature Tm of a polymer crystal?
What factor influences the observed melting temperature Tm of a polymer crystal?
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What change is expected in ΔH and ΔS between Tm and Tmo?
What change is expected in ΔH and ΔS between Tm and Tmo?
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In the equation provided for ΔG, what happens to the term involving side surface energy for lamellar crystals?
In the equation provided for ΔG, what happens to the term involving side surface energy for lamellar crystals?
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What is the effect of short hydrocarbon side groups on Tm in vinyl polymers?
What is the effect of short hydrocarbon side groups on Tm in vinyl polymers?
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What does it mean when ΔG equals zero at the melting point of a crystal?
What does it mean when ΔG equals zero at the melting point of a crystal?
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What is the effect of the presence of the impurity on the melting point of the crystalline phase?
What is the effect of the presence of the impurity on the melting point of the crystalline phase?
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What effect do bulky side groups have on Tm?
What effect do bulky side groups have on Tm?
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What approximation can be made regarding the mole fraction of crystallizable polymer (XA) for small values of XB?
What approximation can be made regarding the mole fraction of crystallizable polymer (XA) for small values of XB?
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According to the equation for Tm, what is true for finite size crystals?
According to the equation for Tm, what is true for finite size crystals?
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What happens to the melting temperature Tm as the number of -CH2- groups increases between amide linkages?
What happens to the melting temperature Tm as the number of -CH2- groups increases between amide linkages?
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What does ΔHV represent in the context of the melting behavior of polymers?
What does ΔHV represent in the context of the melting behavior of polymers?
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In the equation for chain ends in linear polymers, how is the mole fraction of chain ends represented?
In the equation for chain ends in linear polymers, how is the mole fraction of chain ends represented?
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What is a characteristic of random or statistical copolymers?
What is a characteristic of random or statistical copolymers?
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What is the relationship between the mole fraction of the impurity (XB) and the overall crystallization of the polymer?
What is the relationship between the mole fraction of the impurity (XB) and the overall crystallization of the polymer?
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In the modified equation for branches in polymers, how is the term 2/xn adjusted?
In the modified equation for branches in polymers, how is the term 2/xn adjusted?
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What happens to the crystalline structure in block and graft copolymers?
What happens to the crystalline structure in block and graft copolymers?
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Study Notes
Melting in Semicrystalline Polymers
- Several characteristics distinguish polymer melting from other materials' melting
- A single melting temperature isn't possible; melting occurs over a range of temperatures
- Melting behavior depends on specimen history, particularly crystallization temperature
- Melting rate also influences behavior
Estimating Melting Temperature (Tm)
- Tm can be estimated by extrapolation
- Observed Tm (melting temperature) is always greater than crystallization temperature (Tc)
- A plot of Tm versus Tc is typically linear
- Extracting the intersection of the extrapolated line with the Tm=Tc line gives Tm° (theoretical melting temperature)
Dependence on Crystal Thickness
- Observed melting temperature (Tm°) strongly depends on crystal thickness (I)
- Thermodynamics of melting is considered for rectangular lamellar crystals with lateral dimensions (x, y)
- Melting causes a reduction in surface free energy (compensated by increased free energy; AGv).
Annealing
- Heating below Tm increases lamellar crystal thickness
- Driving force is reduction in surface area of the lamellar crystal
- Lamellar thickening happens at relatively high temperatures requiring sufficient thermal energy
- Measured melting temperature depends on the heating rate due to annealing effects; more rapid heating results in lower annealing effects.
Factors Affecting Tm
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Chemical Structure:
- Stiffness of the main polymer chain:
- Rotation about bonds; incorporation of linking groups (e.g., -O-, -CO-O-) affects flexibility; higher flexibility leads to lower Tm
- Presence of a phenylene group increases stiffness and Tm
- Polar Groups:
- Amide linkages (-CONH-) allow intermolecular hydrogen bonding, affecting Tm, and the number of -CH2- groups between the amide linkages affects the degree of intermolecular bonding
- Type and Size of Side Groups:
- Short side groups decrease chain flexibility
- Presence and size of side groups (e.g., methyl groups) affect flexibility and thus Tm
- Long, flexible side groups decrease Tm; bulky side groups increase Tm
- Stiffness of the main polymer chain:
Molar Mass and Branching
- Tm depends on molar mass and branching in polymers
- Chain ends and branches introduce defects in crystals that lower Tm
- Increased molar mass in polymers reduces the effect of chain end influence on Tm
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Description
Explore the unique melting characteristics of semicrystalline polymers in this quiz. Understand how the melting behavior differs from other materials and the factors affecting the melting temperature, including crystal thickness and specimen history. Test your knowledge on concepts like Tm, Tc, and thermodynamics involved in the melting process.
